Bryce Harper left the Philadelphia Phillies' 6-3 loss to the Toronto Blue Jays on Sunday with lower back stiffness.
The Phillies are looking to make the postseason for the first time since 2011.
Harper exited after he struck out with the bases loaded in the seventh inning.
"He'll get a lot of treatment and we'll keep our fingers crossed," Joe Girardi said.
Philadelphia has seven games remaining and entered Monday's action as the No. 7 seed in the National League.