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.
While running out a ground ball to the right side of the infield in the first inning Saturday night against the San Francisco Giants, Harper avoided contact with first baseman Ryder Jones and his left foot slipped on the base. He immediately grabbed his left knee while falling to the ground and writhed in pain behind first base. Harper didn't put any weight on his left leg and had his head down as he was assisted off the field and through the Nationals dugout.
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 24-year-old Harper is a five-time All-Star and was the National League MVP in 2015.
Edwin Jackson (2-2, 3.75 ERA with Washington) of the Nationals was slated to face Chris Stratton (0-2, 6.63 ERA) of the Giants in the pitching matchup Friday.
Harper is now one of the leaders in the NL MVP race and is hitting.326 with 29 home runs and 87 RBI.