The right-hander will return to Boston on Tuesday to see a doctor about the sore right ankle that forced him out of Monday's 1-0 loss to the Toronto Blue Jays.
Beckett left in the fourth inning after feeling pain in his ankle on consecutive pitches to Blue Jays rookie Brett Lawrie, whose 11th inning homer provided the only run of the game.
"It's always concerning," Beckett said. "That's my power leg."
Beckett said he slipped and fell while warming up in the bullpen, but didn't think that had anything to do with the pain that forced him out.
"I felt it on the second-to-last pitch and then it felt a little bit different on the last pitch I threw," he said. "I didn't feel it till those last two pitches.
"It felt like it was locked up and then it popped in and out of the socket or something," Beckett added.
Catcher Jason Varitek didn't notice anything out of the ordinary until Beckett missed with a 1-2 fastball to Lawrie.
"He seemed to make a funny face and kind of came off the mound funny on that pitch," said Varitek, who promptly called manager Terry Francona and trainer Mike Reinold out of the dugout.
After a brief discussion, Beckett walked off the field without assistance and was replaced by right-hander Alfredo Aceves.
"It was getting stiff and it was getting sore so we got him out of there," Francona said.
Beckett was able to stand and walk after the game, but said his ankle was still sore.
"It's bad timing, but who knows?," he said. "I could be back out there in six days. We'll see."
An All-Star for the third time this season, Beckett came in having won his past three starts. He allowed three hits in 3 2/3 scoreless innings with six strikeouts and one walk.
After 10 scoreless innings, Lawrie won it with a two-out drive to center off Dan Wheeler (2-2), his eighth homer since being called up from Triple-A Las Vegas on Aug. 4 and the first walkoff hit of his career.
"My head is still going a mile a minute," Lawrie said. "It's one of those things that's very, very cool and I'll treasure it forever."
Wheeler said he missed with his location on the decisive pitch.
"It was a fastball, just kind of flat," Wheeler said. "It went right down the middle. It was supposed to be down and away but didn't quite get there."
Shawn Camp (3-3) pitched one scoreless inning for the win as the Blue Jays snapped a three-game losing streak.
Boston lost for the fifth time in seven games.
Blue Jays right-hander Henderson Alvarez pitched six scoreless innings, extending his shutout streak to 14. Alvarez allowed four hits, walked one and struck out four.
"He had tremendous movement, just natural movement," Francona said. "The ball was diving all over the place."
Carlos Villanueva replaced Alvarez and walked Kevin Youkilis to begin the seventh. After Carl Crawford flied out, Josh Reddick singled to right and both runners advanced on Varitek's grounder. Marco Scutaro ended the inning by grounding out to shortstop.
Adrian Gonzalez hit a two-out double off Casey Janssen in the eighth and David Ortiz was intentionally walked to bring up Youkilis, who struck out looking.
Toronto put a runner at third base three times in the first four innings. Edwin Encarnacion struck out to end the first, Jose Molina grounded out and Dewayne Wise struck out in the second and, after Aceves walked Lawrie in the fourth, Molina ended the inning by flying out to right.
Aceves left after walking Mike McCoy and hitting Eric Thames with one out in the eighth. Daniel Bard came on and got Jose Bautista to fly out, then struck out Adam Lind.
Jonathan Papelbon struck out Lind on three pitches to leave the bases loaded in the bottom of the 10th and has not allowed a run in 18 innings.
Notes:
Boston recalled C Ryan Lavarnway and RHP Kyle Weiland and purchased the contract of INF Nate Spears, all from Triple-A Pawtucket. Infielder Drew Sutton was designated for assignment to open a roster spot for Spears. … Red Sox LHP Erik Bedard (left knee) will be skipped in his next scheduled start, Friday at Tampa Bay. … Francona said RHP Bobby Jenks (back) is unlikely to pitch again this season. … Blue Jays SS Yunel Escobar (left wrist) missed his second straight game. … Toronto reinstated RHP Dustin McGowan (shoulder) off the 60-day DL. McGowan last pitched in the majors July 8, 2008. … Blue Jays manager John Farrell returned after missing 10 games with pneumonia. … Wise made a superb diving catch on Crawford's sinking liner to center to begin the ninth. … Toronto's five stolen bases were a season-high.