The team announced that Harper is day-to-day with a bruised knee. He was seen walking around with a slight limp in the clubhouse following the team's 4-3 victory.
"It hurts," Harper said. "Whenever you get squared up like that, definitely something that didn't feel good. I think we'll evaluate tomorrow and see how it feels. If I don't feel good, I'm not going to play. If I feel fine, then I'll be in there."
Harper was hit by the pitch in the top of the seventh inning and, after talking with manager Dusty Baker and trainer Paul Lessard, stayed in the game and took first base.
With the Nationals' Daniel Murphy at the plate, Hellickson threw over to first base a couple times, forcing Harper to dive safely back to the bag. But when Murphy hit a line drive to left fielder Tyler Goeddel, Harper was doubled off first without sliding after Goeddel threw a strike to Phillies first baseman Ryan Howard.
Harper was replaced in the field by Chris Heisey in the bottom of the seventh inning.
Harper, the reigning National League MVP, went 0-for-2 at the plate in the series opener between the two NL East foes.
Harper leaves early in Nationals' win
Bryce Harper leaves Monday's game after getting hit in the right knee by a pitch in the seventh inning of Washington's 4-3 win over Philadelphia.
Murphy's two-run single puts Nationals back in front
With the bases loaded, Daniel Murphy singles to right field, driving in two runs to put the Nationals up 4-2.