Alex Brandon/AP

Bryce Harper aggravated a left knee injury sliding headfirst into second base in the first inning of Sunday’s game against the Phillies, and it worsened when he slid into third base and later fouled a ball off it. He hobbled when he walked, hunched over while in right field and lacked his usual burst of speed. Harper left the 6-1 victory in the bottom of the seventh inning and, according to Manager Davey Johnson, could miss a few games. Harper admitted the injury could linger for the rest of the season.

Harper, 20, originally banged up his left knee, among other body parts, when he slammed into the right field fence in Los Angeles on May 13. He missed most of the following two games against the Dodgers, pinch-hitting in one, because of the collision. He played two games in San Diego last week but then missed two more because the knee was still bothering him.

Harper, the Nationals’ best player this season, has played every day since he returned to the lineup on May 20. Harper said after Sunday’s game that he didn’t expect to miss any more time.

“It probably won’t get better until the offseason,” he said. “I just have to deal with the pain and try to keep in there every day and see what happens.”

Harper singled in the first inning off Phillies starter Cole Hamels and, after a throwing error by Freddy Galvis, dove into second base on a close play. Standing following the play, he favored his left knee and hunched over. He stole third base and beat the tag by Michael Young, again sliding headfirst and banging his knees into the ground.

In the third inning, Harper fouled a pitch off his left knee. When he took his position in right, he jogged gingerly. While others tossed the ball around, he was bent over at the waist, his hands on his knees. Even during his at-bats, he hunched over.

“Running, that’s the biggest thing,” he said. “Sometimes when I get a push off my back side it hurts a little bit. It’s more of running part and having that fear of sliding and banging it up. But I don’t really play with fear.”

It was apparent after the first inning that Harper was in pain and struggling to run. He said he didn’t want to leave the game until the Nationals got a lead. He lacked burst while running to first when he grounded into a double play in the sixth inning. Even as he limped, he drew a walk in the seventh inning with a 5-0 lead and was replaced by pinch-runner Roger Bernadina.

“We got a five-run lead and I wanted that fourth A-B and I swing it pretty well against Hamels every time I face him,” said Harper, who was intentionally hit by Hamels with a pitch in his second week as a rookie last season. “I really wanted to stay in there against him. We got that five-run lead and go out.”

Harper avoided trainers in the dugout.

“I just tell them to go away from me,” he said. “I don’t want to talk to them during the game. So if I have a problem, I’ll go to [Johnson]. If I don’t, I’m not going to talk to anyone about it.”

Asked if he had considered putting Bernadina in sooner for Harper, Johnson said: “He’s 20 years old, a great athlete and he heals pretty quick.”

Johnson said Harper wasn’t sent for any tests following the game. He wants Harper to change the way he slides so that he doesn’t expose his knee to further damage.

“If he quit headfirst sliding, he’d be a lot better off,” Johnson said. “But I think he keeps banging that knee and it keeps giving him problems. Hopefully, he’ll be all right. He needs to slide and hit his ass instead of his knee.”

Harper, however, said he knows no other way. He slides primarily headfirst and only when the situation warrants does he slide feet first. Even if he slid feet first, he said, he would do so by tucking his left leg under his right. In his mind, there is no good way to slide on a bum knee.

“I’m going to feel it either way,” he said. “It doesn’t matter. If I slide feet first or headfirst, I don’t think that’s going to help it.”