Boucher left after injuring his knee at the bottom of a pileup in the crease at 4:35 of the second period. Leighton, who had been the starter before he was injured on March 16, made his first career playoff appearance and finished off the first combined shutout in the Flyers' postseason history.
Simon Gagne, who scored in overtime to win Game 4, scored two goals, and Ville Leino had a goal and an assist to help the Flyers force the series back to Philadelphia for a sixth game on Wednesday night. If they win, it's back to Boston for a decisive Game 7.
The 1942 Toronto Maple Leafs and the 1975 New York Islanders are the only NHL teams to come back from a 3-0 deficit to win a best-of-seven playoff series.
Tuukka Rask stopped 27 shots for Boston, which missed a second chance to clinch the series and advance to the conference finals for the first time since 1992.
Leino scored off a rebound in front just 6:41 in to give the Flyers a 1-0 lead. It was still close when Boucher went down.
Boston put a few quick shots on Leighton to test him, but nothing too challenging and the Flyers took a 2-0 lead when Scott Hartnell batted Danny Briere's shot out of the air and into the net with 8:44 left in the second. Bruins forward Steve Begin was off for boarding when Gagne one-timed a centering pass from Mike Richards into the net to make it 3-0 with 2:07 left in the second.
The Boston crowd booed the Bruins off the ice.
And in the third period, it wasn't much better.
Gagne, who missed the first three games of the series with a broken toe, broke free when Dennis Wideman's stick broke at the point and the puck trickled into the neutral zone. Wideman could do little but try to get in Gagne's way, but the Flyers forward easily went around him and in on Rask to make it 4-0 with 13:12 to play.
Boucher wound up beneath teammate Ryan Parent and Bruins forward Miroslav Satan after a scramble for a loose puck. The puck was cleared away, but Boucher remained on the ice; a referee whistled the play dead and quickly waved to the Flyers bench.
After the team's medical staff came out, Leighton immediately stepped onto the ice in a ballcap and began stretching. A few minutes later, Boucher skated off with assistance, favoring his left knee as the Boston crowd gave him polite applause.
Flyers spokesman Zack Hill said that Boucher wouldn't return. The team didn't identify the injury.
Notes
The Bruins won the ratings game in Boston on Friday night, when they went head-to-head with Red Sox-Yankees and Celtics-Cavaliers. According to the NHL, the Bruins averaged an 8.3 local rating on Versus, the Red Sox a 6.3 local rating on NESN and the Celtics a 4.1 local rating on ESPN. That translates to 200,000 households for the Bruins, 153,000 households for the Red Sox and 99,000 households for the Celtics. … The Bruins dedicated a statue of Bobby Orr outside of the TD Garden to honor the 40th anniversary of his Stanley Cup-winning goal against St. Louis. It shows Orr appearing to fly through the air in celebration. … The Bruins had been 5-0 at home in the playoffs.