Manager Dusty Baker said Harper hyperextended his left knee and was scheduled to have an MRI on Sunday.
"We just ask for prayers that it’s not serious," Baker said after his team’s 3-1 victory over the San Francisco Giants that was overshadowed by the injury to one of baseball’s top players. "He was in obvious pain, and he was praying and I was praying, too."
Harper was limping but walking after the game. He was not made available to speak to reporters.
Starter Edwin Jackson was upbeat that Harper was walking around. General manager Mike Rizzo is expected to provide an update on Sunday morning.
"Fortunate that he’s up and walking," Jackson said. "He’s definitely one of those guys that you can’t replace on a team. It’s crazy but it’s definitely great to see him up in spirit and walking around and know that he won’t be significantly out for a period of time or just hurt period. He’s doing great."
While running out a ground ball to the right side of the infield in the first inning, Harper avoided contact with first baseman Ryder Jones and his left foot slipped on the base. His leg buckled , he went airborne and fell to the ground as his helmet went flying.
"I think (starter Jeff Samardzija) said he tried to extend the knee a little bit and try to catch the corner of the bag, and I think he might’ve slipped on the top," Jones said. "It was a little wet. It didn’t look good. I hope he’s all right. It’s just one of those freak plays where you kind of thought there would be a collision and there wasn’t and that might be part of it, too."
Harper clutched at his left knee after going down and was attended to by trainers. The star outfielder put no weight on his left leg as he was helped off the field by hitting coach Rick Schu and head athletic trainer Paul Lessard.
"He looked like he was in pretty good pain," Jones said. "You never want that to happen to a player. … Hopefully he’s all right."
Baker said his first reaction was to think about last September, when catcher Wilson Ramos tore the ACL in his right knee on a noncontact play after a rain delay. Baker said he hopes that’s not the case with Harper.
The game began just after 10 p.m. following a 3-hour rain delay. It was raining steadily when Harper’s injury happened.
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, adding to baseball’s incentive of getting the game in Saturday.
Baker refrained from criticizing the league, saying the teams "were kind of at the mercy of the schedule."
But the game is unlikely to have an impact on the playoff race, as the Nationals lead the NL East by 14 1/2 games and the Giants are out of contention. At the most, it could have an effect on home-field advantage in October.
Even though first baseman Ryan Zimmerman is having an All-Star season, Washington’s title hopes hinge largely on Harper’s health. The 2015 MVP is having another spectacular season, hitting .327 with 29 home runs and 87 RBIs while playing in 105 of the Nationals’ first 113 games.
Harper didn’t last long in his 106th game and was replaced by Andrew Stevenson, who took over in centre field with Brian Goodwin moving to right. Despite many fans waiting out the delay, the stadium went eerily silent in the minutes after Harper’s injury.
Because of a bevy of injuries, the Nationals have used 12 outfielders this season. They have been without Adam Eaton since late April because of a torn ACL in his left knee that ended his season, Jayson Werth since early June because of a bruised left foot and Michael Taylor since July with a right oblique strain.
The Nationals are running away with the division despite those injuries and more to starters Stephen Strasburg and Joe Ross, relievers Shawn Kelley and Koda Glover and shortstop Trea Turner, but a long-term injury to Harper would be far more trouble to Washington’s post-season hopes.
Harper is under contract through next season and can be a free agent in the winter of 2018.
"The good thing is he’s young and strong," Baker said. "I’m very optimistic that he’ll be at least fine shortly."