Harper crumpled to the ground, where he remained for several minutes, after slipping on the slick outfield grass while attempting to make a throw home.
"He's sore," manager Matt Williams said before Friday's game. "He hasn't done a whole lot today. He's getting treatment. I'm sure at some point he'll get some swings in and do some light stuff."
The 22-year-old phenom leads the Nationals in almost every offensive category, including home runs, RBIs, runs scored and on-base percentage. He is currently hitting .344 and entered Friday leading the Majors in OBP (.476), slugging percentage (.720) and walks (54). Harper has also already matched his previous career high for homers in a single season with 22.
The Nationals were confident that Harper had avoided a serious injury following Thursday's game, though the outfielder suggested he could sit out Saturday to rest the hamstring. It's unclear at this point whether Harper will return for Saturday's game, though the club has not strayed from his status.
Paul Casella is a reporter for MLB.com. Follow him on Twitter at @paul_casella. Jacob Emert contributed to this story. This story was not subject to the approval of Major League Baseball or its clubs.