Harper aggravated his sore left knee three times in Sunday's 6-1 win over the Philadelphia Phillies, twice with head-first slides in the first inning. He slid into second on an infield hit-and-error, then hit the dirt again with a steal of third.
For good measure, he then fouled a pitch off the knee in the third inning. He was limping throughout the game and was removed for a pinch runner after drawing a walk in the seventh.
"If he'd quit head-first sliding, he'd be a lot better off," Johnson said. "He'll probably be down a few days. I don't know if there's fluid on the knee or what, but he was certainly gimpy."
Harper, however, quickly answered "no" when asked if he expected to miss any playing time.
"It probably won't get better until the offseason," he said. "I've just got to deal with the pain. ... I could rest it for four days and feel better and I could go out there and slide and bang it up again - and back to square one."
Harper also differed with the manager on whether it would do any good to stop sliding head-first, saying the knee feels pressure either way.
"If I slid feet-first right now, my knee might pop and not be good," Harper said.
Harper has already missed three games because of the knee that's been bothersome since he ran into the wall in a game against the Los Angeles Dodgers on May 13, a collision that also left him with a gash in the chin that required 11 stitches.
Harper is hitting .287 with 12 home runs and 23 RBIs.
Johnson also announced that left-hander Ross Detwiler will miss a second start and go on the 15-day disabled list because of a strained oblique muscle.
Johnson said the muscle is still tight, so Detwiler will skip Tuesday's scheduled start. His placement on the DL will be retroactive in the hopes that he can return to the rotation next Sunday.
Left-hander Xavier Cedeno will be recalled from Triple-A Syracuse. Johnson said he hasn't decided who will start Tuesday.