Doolittle, 32, allowed three home runs and four runs in the ninth inning Saturday night against the Milwaukee Brewers to blow a three-run lead for the Nationals. Washington eventually lost the game 15-14 in 14 innings.
"He has always had knee issues," Nationals manager Dave Martinez said Sunday, according to the Washington Times. "He can't get on that front side like he wants to (on his delivery). I told him, 'You are not letting anybody down. But we have to get you right.' It came to a head."
Martinez said Sunday that he he will play matchups with his bullpen in the ninth inning during Doolittle's absence. But he made it clear that Doolittle will not lose his job.
"When he does come back, he's our closer," Martinez told reporters.
"I'm still searching for answers," Doolittle said after Saturday's game. "I don't know. ... It just wasn't coming out tonight. That part of the order, that team, there's just nowhere to hide."
Right-hander Kyle McGowin was recalled from Double-A Harrisburg in a corresponding move.
The Nationals (67-56) currently hold the first wild-card spot in the National League, with a 2-game lead over the Chicago Cubs.
The Associated Press contributed to this report.