Bryce Harper injured his knee rounding first base Saturday night and had to be helped off the field. The right fielder lunged for first base and slipped on the bag, and his leg appeared to buckle awkwardly.
Harper clutched at his left knee after going down and was attended to by trainers.
Harper had to be carried off after not being able to put any weight on the leg.
Saturday's game started after a three-hour rain delay in D.C., and it's unclear how much the damp conditions affected the basepaths.
The last-place Giants (46-70) are making their only trip of the year to Washington. The Nationals and Giants are set to play a doubleheader on Sunday. He played in 105 of Washington's first 113 games. It was reminiscent of the scene in the same area behind first base on April 29, when Nationals outfielder Adam Eaton sustained a season-ending knee injury, also trying to beat out a groundball.