Beckett will fly back to Boston on Tuesday and have his ankle checked out by the team's foot specialist, Dr. George Theodore.
The righty felt discomfort in his ankle on his second to last pitch. And after his final offering to Brett Lawrie, Beckett was limping around.
"I felt it on the second to last pitch, and then it felt a little bit different on the last pitch," said Beckett after the Sox's 1-0, 11-inning loss to the Blue Jays. "I did fall in the bullpen trying to get off the mound and take a shirt off. I didn't feel it between there. I didn't feel it until those last two pitches."
After a brief visit from manager Terry Francona and trainer Mike Reinold, it was clear Beckett had to exit the game.
Though there was no way of knowing for sure, Beckett thinks his bullpen fall was unrelated to his ankle injury.
"I busted my [behind and] fell," said Beckett. "I was just trying to change shirts and they have concrete or something down there. I fell. I don't know, but I don't think it had anything to do with it. I think I would have felt it earlier than that."
How concerned is Beckett about his ankle?
"I don't know, like a scale? It's always concerning," Beckett said. "That's my power leg."
Even before the game, Francona said the Red Sox would try to give Beckett an extra day of rest before his next two starts. Beckett is 12-5 with a 2.49 ERA.
"We're going to send him back tomorrow morning, try to get him looked at obviously extensively to see what happened, what's going on there," Francona said. "We really don't know. It was getting stiff, it was getting sore, so we got him out of there. Now we'll see what's going on in there."
With Beckett one of the key components of a team that hopes to win the World Series, there was some concern in the clubhouse after Monday's game.
"Obviously he's been our best guy from Day 1," said setup man Daniel Bard. "Talking to him, it didn't seem as bad as we first thought. I really don't know what's wrong with him. Obviously we need him down the stretch."