(Chris Gardner/Getty)

Bryce Harper will fly back to Washington on Sunday for further tests on his ailing left hip, which the Nationals fear may be injured more seriously than initially believed.

Harper aggravated his hip taking batting practice 10 minutes before first pitch Saturday night, and the hip pain left him floored in the batting cage adjacent to the Nationals’ clubhouse.

“It’s something we need to aggressively attack,” Manager Davey Johnson said. “We’ve been kind of treating it as a slight hip strain. But obviously, it’s something more serious than that. We need to fix it, and he needs to finish the year strong.”

When asked if Harper may miss the Nationals’ final 21 games, Johnson replied, “I don’t know.”

Harper will see Nationals medical director Dr. Wiemi Douoguih. Harper is scheduled to rejoin the Nationals in New York for a four-game series against the Mets starting Monday. The Nationals are not yet fully certain of the severity of the injury.

Harper, who has battled injuries since colliding with the right field wall in Atlanta in late April, began feeling pain in his hip dating back to at least the start of the week. Harper sat out Wednesday in Philadelphia. After a day off, Harper talked his way into the Nationals’ lineup Friday night. Harper was originally in the Nationals’ lineup Saturday, too.

“He didn’t need to tell me anything,” Johnson said. “He was on the ground in the cage after he swung at a pitch. I guess it was about 10 minutes before the game started. I was in a rush to bring a new lineup out there.”

Wednesday, Nationals head athletic trainer Lee Kuntz referred to Harper’s ailment as “soreness.” Friday, Johnson labeled it a “hip strain.” Saturday night, Johnson said Harper was now feeling inflammation in his hip.

Johnson faulted Harper’s extensive pregame hitting regimen.

“He aggravated it swinging,” Johnson said. “He comes and hits early and then he hits in BP and then he comes back in here and hits more. Hopefully, he’ll figure out what it is and treat it and he’ll meet us in New York.

“I was a little upset that he keeps swinging instead of just resting and playing. He’s still young and exuberant and has his own routine. Hopefully he’ll get fixed and be fine in New York.”

Harper has struggled through injuries all year, but when healthy he has managed a near-historic performance. His .882 OPS would rank ninth all-time among players 20 or younger, and he ranks third on the Nationals with 19 homers.

Harper missed 31 games, including all of June, with bursitis in his left knee, a product of his brutal collision with the right field wall at Dodger Stadium. He played off-and-on for 11 days through that injury before the Nationals finally shut him down.