Bryce Harper left the Phillies' series finale against the Nationals after the fourth inning Thursday night with a mid-back spasm.
He underwent further evaluation during the game and is day to day, according to manager Rob Thomson.
Thomson said neither he nor Harper knew how he suffered the injury. The team will reevaluate him Friday.
Harper's exit came an inning after he took a baseball to the face. Dom Smith grounded a ball softly up the first base line with one out in the fourth inning and Harper had a simple play if he gloved it and stepped on the bag. The ball bounced up from the bag and struck him in the face, resulting in Smith reaching safely. The play was initially ruled an error on Harper but changed to a single.
Harper appeared to be in some discomfort later in the inning on a pick-off attempt by Aaron Nola. He caught the ball and reached across his body for the tag, then shook his right arm after the play.
“I don’t know where it happened. He doesn’t know where it happened, I don’t think,” Thomson said. “They came to me between innings and said it had stiffened up. That was it.”
Alec Bohm shifted over from third base to first and Rodolfo Castro replaced Harper in the three-spot.
Harper went 0-for-1 with a walk. He's hit .311 with a .385 on-base percentage, seven extra-base hits, eight RBI and eight runs scored in his last 12 games. Six of his last 10 starts have come at first base.