"He aggravated [it] hitting, swinging," Johnson said following Washington's 9-2 win over the Marlins Saturday night. "He comes and hits early, then he hits in BP -- hit a great BP -- and then he goes back and hits more. I think he's got some inflammation in that hip."
Harper, who has been battling the injury and receiving treatment for it since Monday, was pulled from the lineup 10 minutes before the game when Johnson was told Harper was on the ground in the cage following a swing.
The reason for the injury remains unknown. Harper missed more than a month of the season when he banged his left knee into a wall.
Asked whether it could be an injury that ends Harper's season, Johnson said he didn't know. The second-year player is batting .273 with 19 home runs and 49 RBIs.
"It's something we need to aggressively attack," Johnson said. "I don't know. We've been kind of treating it as a slight hip strain, but obviously this is something more serious than that. We need to fix it and he needs to finish the year strong."
Harper got two days' rest on Wednesday for the series finale against the Phillies and on Thursday's off-day.
In his return to the lineup on Friday, Harper went 0-for-2 with a walk. Harper had said to Johnson before Friday's opener in Miami that he was not 100 percent, but that he still wanted to play.
"I was a little upset that he keeps swinging, you know?" Johnson said. "Instead of just resting it and going out there playing. But he's still young and exuberant and has his own routine. Hopefully it gets fixed and [he'll] be fine in NY."
Christina De Nicola is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.