The 25-year-old Sierra threw the ball back into the infield as Blue Jays trainer George Poulis ran onto the field. Sierra was removed, but was able to leave the field without any assistance under the watchful eye of Poulis and manager John Gibbons.
The injury could be yet another blow to the club's outfield situation. Center fielder Colby Rasmus is done for the season after being struck in the face by an errant warmup throw from Anthony Gose, while Rajai Davis is expected to leave the team soon for the birth of his first child.
That leaves Kevin Pillar and Gose as potentially the only available outfielders with five games remaining. Gibbons previously said utility man Mark DeRosa or infielder Munenori Kawasaki could play the outfield in an emergency situation.
"If he heals pretty quick," Gibbons said when asked if Sierra would be available when Davis departs the team. "Or we might have to get somebody here, I don't know."
It's not immediately clear who the Blue Jays would potentially promote because all of their affiliates are done for the season. The club would need to add someone to the 40-man roster because all of its outfielders on that list are injured or already on the big league roster.
Gregor Chisholm is a reporter for MLB.com. Read his blog, North of the Border, and follow him on Twitter @gregorMLB. This story was not subject to the approval of Major League Baseball or its clubs.