This certainly seems like the type of injury that could mean the end of Harper's 2017 campaign. Harper fell to the ground and held his left knee for several minutes after making the second out.
It had been raining for most of the night, as the game didn't start until around 10 p.m. E.T. following a three-hour rain delay. 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.
Major League Baseball was in control of weather decisions because it's the Giants' final trip to Washington this season. The series opener Friday was postponed after a lengthy rain delay to be a part of a day-night doubleheader Sunday.
He is among the National League's leaders in batting average, home runs and RBIs this season. He played in 105 of Washington's first 113 games.
Rookie Andrew Stevenson came on to play center field in the second and Brian Goodwin moved from center to right. 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.
Harper is under contract through next season and can be a free agent in the winter of 2018.