Harper left Philadelphia's 6-3 loss Sunday to the Toronto Blue Jays with lower back stiffness, and any absence by the slugging right fielder could sap the Phillies' bid for their first playoff berth since 2011.
Harper exited after he struck out with the bases loaded in the seventh inning. He took a slow walk in the dugout toward the clubhouse and signaled to manager Joe Girardi he was done for the day. Phil Gosselin took over for Harper in right field.
Girardi said Harper will receive treatment but it was unknown if Philadelphia's franchise player would play Monday in Washington. Girardi said Harper's back bothered him earlier this season.
"He'll get a lot of treatment and we'll keep our fingers crossed,'' Girardi said.
Harper, 27, has 11 homers and 28 RBIs for the Phillies (27-26), who hit the road for the final seven games.
The Associated Press contributed to this report.