After a couple of minutes on the ground, he was helped off the field with his legs hanging in the air.
Nationals general manager Mike Rizzo said Sunday that an MRI revealed no structural damage is Harper's knee.
The start of the game was delayed 3 hours, 1 minute by rain Saturday and there was a light rain in the top of the first inning.
There's no definitive timeline for Harper to return, but Rizzo is hopeful the star outfielder and NL MVP candidate will be back before the end of the season.
The news is perhaps the best possible for the Nationals after Harper's injury on Saturday night, as his foot slipped off a wet first base as he was trying to beat out an infield single, and it slid awkwardly onto the infield directly below. He was not made available to speak to reporters.
I'm no doctor, but this injury does not look good at all for Bryce Harper... Washington (69-45) has a 14.5 games lead on the Miami Marlins in the National League East.
Keep in mind, last year, catcher Wilson Ramos tore his ACL against the Arizona Diamondbacks in late September in a game that had a rain delay involved.
It was the first time the Giants and Nationals crossed paths since the late May series at AT&T Park, and the benches-clearing incident that was window dressing for a one-on-one confrontation between Harper, right-hander Hunter Strickland and the reliever's three-year grievance. If Harper has that same injury, that would devastate the team's chances of reaching their ultimate goal of a World Series championship.
Because of a bevy of injuries, the Nationals have used 12 outfielders this season.
Harper, 24, entered the game hitting.326 with 29 home runs and 87 RBIs this season. The Nationals don't have another Harper in the system, and neither does anyone else in baseball.
Who would replace Harper in right field and in the lineup?