Harper was unable to put weight on his leg and was carried off the field by his hitting coach and the team trainer.
General manager Mike Rizzo told reporters Sunday an MRI revealed that Harper does not have any ligament or tendon damage in his injured left knee, and the Nationals are optimistic he will return before the end of the season.
After waiting three hours to start tonight's game, the Washington Nationals may have just had the worst news possible.
Harper is hitting.326 with 29 homers and 89 RBIs in 106 games and 402 at-bats for the first-place Nationals, who entered the day 14 1/2 games ahead of the second-place Miami Marlins in the National League East.
"When talking about building an organization, you'd be short-sighted you'd be short-sighted to think about the 25 men on the major league roster", Rizzo said.
There has been no update on Harper's injury.
Harper struck first base awkwardly while running out a grounder and immediately fell to the ground in pain after appearing to slip on top of the bag. The Nationals' training staff and manager Dusty Baker rushed to the field to help Harper off, as Harper was unable to walk off under his own power. Harper is slashing.327/.420/.616 on the season.
"He's definitely one of those guys that you can't replace on a team".
San Francisco Giants' Gorkys Hernandez, right, steals second against Washington Nationals shortstop Wilmer Difo (1) during the fourth inning of the first baseball game of a split doubleheader, Sunday, Aug. 13, 2017, in Washington. Despite many fans waiting out the delay, the stadium went eerily silent in the minutes after Harper's injury. Melancon has been on the disabled list with a right forearm strain since June 28 and had two strikeouts in an inning during a rehab assignment with Class-A San Jose on Wednesday.