The Nationals tried to give Harper time to heal without using the disabled list. Manager Davey Johnson said the move became necessary when the team needed help in the bullpen before Saturday night's game against the Braves.
Johnson also said the team decided it was necessary to protect Harper's status for the season.
"You always do what's best for the player," Johnson said.
"If the club felt that he would be able to come back in three or four or five days, we wouldn't have DL'd him."
Harper has missed the last five games and has been bothered by the knee since he crashed into the wall in a May 13 game at the Los Angeles Dodgers. The move was made Saturday retroactive to May 27.
The Nationals already are without outfielder Jayson Werth, who has been on the disabled list since May 3 with a right hamstring injury. The team is off Monday and Werth could return Tuesday when Washington opens a home series against the Mets.
The 20-year-old star is batting .287 and is tied for fifth in the NL with 12 homers. He has 23 RBIs and two stolen bases.
"I think, in talking to Bryce, we've got to get by this where it's not a recurring thing," Johnson said. "He's not going out there playing 80 per cent. His legs are just as big a part of his game. Taking that extra base, running hard. I don't want to take a chance of him getting injured further."
Johnson said Harper probably aggravated the injury with his aggressive play.
"You don't know how bad the injuries were," Johnson said. "He felt pretty good the next day. He had some bruises but he didn't have any swelling. When he went back to paying, he dove for a ball, then slid head-first. I think it was probably a sensitive area and he exacerbated it by pounding on it more. He felt good enough that he wanted to play and he knows his body better than anybody."
Harper was with the team but did not speak with reporters before Saturday night's game.
The Nationals recalled right-hander Erik Davis from Triple-A Syracuse. The team needed bullpen depth after starter Stephen Strasburg lasted only two innings on Friday night due to a strained right side.
Davis was 1-2 with a 3.00 ERA and seven saves at Syracuse. He has no major league experience.