Marty Turco made his Bruins debut and was solid, stopping 20 of the 22 shots he faced in two periods and keeping Boston in the game.
After finally winning back-to-back games for the first time in two months, the Bruins have now lost back-to-back games.
The Penguins have won nine in a row and have pulled to within two points of the Rangers for the top spot in the East, with the Rangers playing the Islanders later on Sunday.
The Bruins remain just two points up on Ottawa for the Northeast Division lead, and now have just two games in hand on the Senators. Boston continues its three-game road trip Tuesday at Tampa Bay.
Third Period, 18:08, Penguins 5-2: The Bruins will now be shorthanded as Johnny Boychuk drive hard to the net but crashes into Fleury and is called for goalie interference.
Third Period, 16:07, Penguins 5-2: The Penguins add an insurance goal to put this one away as Pascal Dupuis scores on a breakaway.
With Chara pinching in deep, Lucic was up b the blue line, where Dupuis slipped the puck past him and was off to the races, finishing the bid with a backhander.
Third Period, 15:29, Penguins 4-2: The Bruins can't convert that power play, with Krejci robbed early by Fleury on a bid for a hat trick at the right post.
Third Period, 13:29, Penguins 4-2: The Bruins will have a power play, but lose another player as Johnny Boychuk goes off groggy after being hit from behind into the boards by Evgeni Malkin. Malkin received just two minutes for boarding in what could have easily been a major penalty.
Third Period, 13:13, Penguins 4-2: Time is becoming as big an enemy to the Bruins as the Penguins, who have taken a page out of Washington's book with a few icings here in the third.
Third Period, 9:43, Penguins 4-2: Turco continues to do his part as the save totals continue to climb, but the Bruins need to get more going offensively to cut into this deficit as the midway point of the third period approaches.
Third Period, 4:50, Penguins 4-2: Turco comes up with another big sequence of stops in a flurry around the Boston net. He stops Craig Adams on a couple whacks in front, then turns aside a long-range blast by Brooks Orpik.
Third Period, 2:09, Penguins 4-2: Turco comes up with another huge save, stopping Joe Vitale as the Penguins forward breaks in all alone behind the Bruins defense after taking the puck away from Chris Kelly.
Third Period, 0:00, Penguins 4-2: The final frame is under way in Pittsburgh, where the Bruins will try to rally from a two-goal deficit. Bergeron is back on the bench to start the period after leaving in the second.
Second Intermission Notes: The Bruins turned in a better effort in the second, but still are down two goals. More importantly, they're also down three players with Adam McQuaid, Max Sauve and Patrice Bergeron all suffering injuries. McQuaid and Sauve will not return in this game, while Bergeron's status is uncertain.
With the short bench, the Bruins have gotten all of their production from the top line, with David Krejci scoring both goals and Milan Lucic and Tyler Seguin each adding two assists. That trio has combined for eight shots (4 by Krejci) as Boston now has a 23-21 edge in that category after a 16-11 advantage in the second.
Boston also has improved its physical play, outhitting Pittsburgh 7-3 in the second. Andrew Ference is up to 13:19 in ice time in his first game back, with a shot, a hit and a plus-1 rating.
Marty Turco's first appearance as a Bruin is off to a strong start. He stopped 10 of the 11 shots he faced, including a highlight-reel save on Jordan Staal just before Boston's second goal. He has looked sharp and has helped keep the Bruins in this one despite their poor start and the mounting injury problems.
End Second Period, Penguins 4-2: The middle frame comes to a close. The Bruins outscored Pittsburgh 2-1 in that period, but still face an uphill climb in the third trailing by a pair of goals.
Second Period, 18:17, Penguins 4-2: The Bruins nearly cut it to one as Chris Kelly and Brad Marchand break in on a 2 on 1, but Fleury makes the stop as Kelly shots from the left wing.
Second Period, 16:40, Penguins 4-2: The Bruins have announced that Max Sauve will also not return to this game, ending his NHL debut early. No word on whether Bergeron will be back.
Second Period, 14:45, Penguins 4-2: The Bruins are now down three players, as Max Sauve has joined McQuaid and Bergeron in the room with an unknown injury. The Boston bench has plenty of room right now.
Second Period, 13:57, Penguins 4-2: The Penguins nearly push the lead back to three again, but Tyler Kennedy's shot from the right slot rings iron.
Second Period, 10:01, Penguins 4-2: The Bruins cut the deficit to two once again, and once again it's David Krejci with the goal.
Milan Lucic set it up by moving around behind the net and feeding it out to Krejci at the right post.
Second Period, 9:08, Penguins 4-1: Turco makes his best stop since coming on to start this period, robbing Jordan Staal in front.
Second Period, 8:26, Penguins 4-1: Tempers flare a fit with Pittsburgh defenseman Ben Lovejoy shoving with Brad Marchand and Chris Kelly. No penalties called, but there will likely be much more of that as the Bruins' frustration mounts.
Kelly was on the ice there because Bergeron again headed back to the locker room.
Second Period, 7:15, Penguins 4-1: The Bruins finally create another rare chance in the Pittsburgh zone, and Fleury flashes the glove out to rob Dennis Seidenberg on a shot from the left side.
Second Period, 4:27, Penguins 4-1: The Penguins get that goal right back as Chris Kunitz scores to push the lead back to three.
Turco had no chance on that one, as Malkin went around behind the net and sent the puck out to Neal in the left circle. Neal then shuffled the pass across to Kunitz, who blasted a one-timer into the open side of the net from the right slot.
Second Period, 3:15, Penguins 3-1: A good sign for the Bruins. Bergeron is back on the ice, not missing a shift after briefly going to the room in pain after blocking a shot.
Second Period, 1:56, Penguins 3-1: The Bruins show some life as David Krejci gets Boston on the board after returning to even strength.
Krejci came in on a clean breakaway and beat Fleury top shelf to cut the deficit to two goals.
Second Period, 1:05, Penguins 3-0: More bad news for the Bruins, as Patrice Bergeron heads to the room in pain after blocking a shot on the penalty kill.
Second Period, 0:00, Penguins 3-0: The middle frame is under way in Pittsburgh, where the Bruins remain on the penalty kill and Marty Turco is now in goal, making his Bruins debut in relief of Tim Thomas.
First Intermission Notes: Things couldn't have gone much worse for the Bruins in that opening period. they were outplayed, outhit and outworked as Pittsburgh jumped out to a 3-0 lead.
Even worse, they lost defenseman Adam McQuaid in the process. he was injured hitting James Neal just before Pittsburgh's second goal, and the Bruins have announced McQuaid will not return.
The Penguins outshot Boston just 10-7, but had the better of the chances. They were also more physical, with a 12-4 edge in hits. Boston started slow and fell behind 2-0 quick, started to turn things around, but frustration settled in late with three penalties in the final 2:44 leading to two 5-on-3 chances and a third Pittsburgh goal.
Max Sauve played five shifts for 3:12 in his first period of NHL action. Andrew Ference played 6:23 in his first game back, and will need to play even more in the final two periods with McQuaid out. the usually reliable fourth line is a combined minus-6 so far with that unit out for both of Pittsburgh's ecven-strength goals.
End First Period, Penguins 3-0: A brutal opening frame comes to a merciful close for the Bruins, but not before Pittsburgh jumped out to a commanding three-goal lead.
First Period, 19:35, Penguins 3-0: The Bruins take another penalty with Dennis Seidenberg tripping Malkin. That will give Pittsburgh another brief 5 on 3.
First Period, 18:48, Penguins 3-0: The Penguins take advantage of the 5 on 3 as James Neal extends the lead to three.
Evgeni Malkin sent the puck down from the right point to Neal, who drove down the left slot and fired a wrister past Thomas.
First Period, 17:51, Penguins 2-0: The Bruins do pick up a second penalty now, with Zdeno Chara joining Lucic in the box for interference as Pittsburgh will have a lengthy 5 on 3.
First Period, 17:16, Penguins 2-0: The Bruins will be shorthanded as Milan Lucic picks up a slashing call in front as frustration starts to set in. Boston could have had another call as Krejci collided with Fleury behind the net.
First Period, 16:10, Penguins 2-0: The Bruins doing a much better job of pressuring the Penguins, keeping the puck down in Pittsburgh's end of the ice and creating some offensive chances. It hasn't produced a goal yet though, and the Bruins need some results from their improved effort after spotting the Penguins a two-goal lead.
First Period, 12:23, Penguins 2-0: The Bruins finally creating some chances as David Krejci scoops up a giveaway by Brooks Orpik and sends in a shot with Milan Lucic in front. Tyler Seguin also has a bid from the left slot as that line created multiple chances on a solid shift.
First Period, 10:33, Penguins 2-0: The Bruins are continuing to be outplayed, with Pittsburgh getting the better of the hitting early with Engelland nailing Jordan Caron on one memorable hit. Boston also was fortunate to avoid its first penalty when Brian Rolston got away with a hold on Matt Cooke. Overall, not close to the start the Bruins needed in this one.
First Period, 7:12, Penguins 2-0: The Penguins strike again as Matt Niskanen fires in a point shot through a screen in front.
James Neal and Chara were in front of Thomas and the puck appeared to deflect in off Neal, but so far has been credited to Niskanen.
Making matters worse was just before the goal Adam McQuaid was hurt throwing a hit on Neal in the corner and went off in pain.
First Period, 6:25, Penguins 1-0: The Bruins can't take advantage of that man advantage as the Bruins power play continues to sputter, now 1-19 over the last seven games.
First Period, 4:25, Penguins 1-0: The Bruins will get a chance to tie the game on the power play as James Neal is sent off for interference after a hit on Zdeno Chara.
First Period, 3:24, Penguins 1-0: The Bruins surrender the first goal once again, with Arron Asham putting the Penguins on the board.
Ben Lovejoy took the shot from the point that Shawn Thornton partially blocked, but Asham pounced on the loose puck in front and fired it home.
First Period, 1:27, 0-0: The Bruins nearly strike first as Tyler Seguin makes a nice rush down the right side and just misses connecting with Milan Lucic on a crossing pass with Lucic staring at an open net from the left slot.
First Period, 0:00, 0-0: And the action is under way in Pittsburgh, where teh Bruins will look to avoid back-to-back losses this weekend and end the Penguins' eight-game win streak.
12:35 p.m.: The Bruins will open this one with Brad Marchand, Patrice Bergeron and Jordan Caron up front, with Zdeno Chara and Dennis Seidenberg on defense and Tim Thomas in goal.
The Penguins counter with Matt Cooke, Joe Vitale and Tyler Kennedy up front, Matt Niskanen and Deryk Engelland on the blue line and Marc-Andre Fleury in net.
12:30 p.m.: Both teams will be getting injured defensemen back in this one, with Andrew Ference returning for Boston and Deryk Engelland back for Pittsburgh.
With Ference back, Greg Zanon is out for the bruins, who also scratch Mike Mottau, Daniel Paille (upper body) and Benoit Pouliot (lower body).
Paul Martin, Richard Park, Dustin Jeffrey, Brian Strait, Kris Letang, Brent Johnson and Sidney Crosby are out for the Penguins.
12 p.m.: Tim Thomas leads the Bruins out for warm-ups and will start yet again for the Bruins. That's the ninth straight game and fifth start in a row for Thomas.
Marc-Andre Fleury leads the Penguins out and will start for Pittsburgh. He's 6-0-0 with a 1.14 GAA in his last six starts.
11:50 p.m.: Max Sauve will make his NHL debut for the Bruins in Pittsburgh, as Benoit Pouliot will remain out with a lower-body injury.
Andrew Ference could return for Boston. he will skate in warm-ups and be a game-time decision after missing the last five games with a lower-body injury.
Daniel Paille (upper body) skated in the morning and will remain out. Mike Mottau also skated and appears likely to be a healthy scratch.
Bruins coach Claude Julien would not reveal his starting goalie before the game. It's possible Marty Turco could make his Bruins debut, but Julien could also continue to ride Tim Thomas, who has appeared in eight straight games and started the last four since Tuukka Rask was injured.
11:30 p.m.: The Bruins and Penguins will both have plenty at stake when they clash in this Sunday matinee.
The Bruins are looking to hold off Ottawa and secure the Northeast Division title and second seed in the East. The Senators pulled within a point of Boston after a shootout loss to Buffalo on Saturday, but the Bruins still have three games in hand on Ottawa.
The Penguins have rattled off eight straight wins and are suddenly very much within range of catching the Rangers for the Atlantic Division lead and the top seed in the East. Pittsburgh begins the day just four points behind New York, which faces the rival Islanders on Sunday.
8 a.m. ET: The Bruins won back-to-back games for the first time in nearly two months with victories over Toronto and Buffalo last week. But they couldn't extend the run to three Saturday against Washington.
After losing 4-3 to the Capitals, the Bruins will look to start a new streak in Pittsburgh on Sunday as they open a three-game road trip against the streaking Penguins (41-21-5, 87 points).
The Bruins (40-24-3, 83 points) may be without Benoit Pouliot, who was a late scratch on Saturday with a lower-body injury. Max Sauve was recalled to accompany the team on the trip after defenseman Mike Mottau played forward against Washington, but forward Daniel Paille (upper body) and defenseman Andrew Ference (lower body) are close to returning to the lineup.
Boston won't get much sympathy from the Penguins, who have dealt with plenty of key injuries themselves. None have been bigger than the loss of Sidney Crosby, who has been out with a concussion since the Bruins' last trip to Pittsburgh all the way back on Dec. 5.
Crosby was cleared for contact this week and there were hopes that he would make his return on Sunday, but he stated Saturday that he is not ready to play just yet. That's a disappointment to the Penguins and hockey fans all over, and even the Bruins were a little disappointed by the news.
"You want to play against the best players in the world," Bruins defenseman Johnny Boychuk said. "He's one of them, but they still have a really good hockey club and we have to be ready for them. They've still got a lot of skill and a lot of grit there, and good goaltending as well."
Pittsburgh has shown how much skill it has without Crosby, winning its last eight games by a combined 30-11 count to pull within four points of the Rangers for the top spot in the East.
The puck drops at 12:30 p.m., so check back here for updates on all the action.