Harper suffered a bone bruise injury in his knee during the play in Saturday's game.
He was facing San Francisco Giants righty Jeff Samardzija on the play. Samardzija tossed in an 0-2 offering, which Harper pulled to the right side. Giants first baseman Ryder Jones dove and stopped the play. He then began a foot race against Harper. Jones reached the base first for the out, but as soon as Harper's left foot touched the bag, he went airborne. His legs twisted awkwardly underneath his body and he catapulted past the base, landing in the dirt. Harper clutched his left knee in agony as trainers came out to assist.
Nationals manager Dusty Baker came out of the dugout with his hands sitting on his cap in disbelief.
"We just ask for prayers that it's not serious," Baker told reporters after the game, according to MLB.com. "We're just urging everybody out there that are Nationals fans and baseball fans to say a prayer for him tonight because he was in obvious pain out there. We're just hoping for the best."
Nationals general manager Mike Rizzo said Sunday that an MRI revealed no structural damage is Harper's knee.
"Although we feel we've dodged a bullet a bit here with any long-term ligament and tendon damage, the bone bruise is something of significance," Rizzo said, according to the Washington Post. "And we're going to treat him cautiously and hopefully have him back later on this season."
The Nationals currently have "no timeline whatsoever" for Harper's return, according to Rizzo. Washington (69-45) has a 14.5 games lead on the Miami Marlins in the National League East.
Harper, 24, is a National League MVP candidate. The 2015 National League MVP is hitting .326 with 29 home runs and 87 RBI. He has a 4.6 WAR.
Washington won the game 3-1. Andrew Stevenson replaced Harper in the team's lineup.