The Washington Nationals placed closer Sean Doolittle on the injured list with right knee tendinitis Sunday morning, hours after the left-hander blew his sixth save of the season Saturday night against the Milwaukee Brewers.

“Hopefully, it won’t take as long,” Manager Dave Martinez said. “When he does come back, he’s our closer. And I reiterated that to him. He’s our closer, but we got to get him right.”

The manager isn’t sure how he will close games in the meantime. He might play the matchups or ride the hot hand, but it doesn’t sound as if there will be an audition for a replacement in the ninth inning. He emphasized the new healthy relievers acquired at the deadline to overhaul the bullpen — right-handers Daniel Hudson and Hunter Strickland — have pitched a lot and he needs to protect them for the rest of the season as well.

Martinez noticed the knee issues after watching video Saturday and talking to the medical staff, he said. Martinez felt like Doolittle couldn’t land properly on his front side, and the two talked Sunday morning. Martinez was concerned Doolittle might compensate for the knee and hurt something else.

“I told him, ‘You’re not letting anybody down, but we got to get you right,’ ” Martinez said. “He wanted to pitch; he wants to pitch. I said: ‘I want you to pitch, and I want you to pitch the ninth inning in close games for us. But I want to make sure that you’re healthy, and that’s our big concern.’ ”

The next step for Doolittle is rest. The Nationals want him to keep throwing and keep his arm in shape, but they’re making it a priority for him to minimize stress over the next 10 days.

The 32-year-old has been used at a higher rate this season than he has in his career. He leads the majors in games finished (49) and was on pace for a career-high 72 appearances — topping the 61 he made for the Oakland Athletics in 2014. When pressed on his usage of Doolittle — including three back-to-back appearances since Aug. 1, when the team addressed its bullpen woes and seemingly diffused concerns of overuse — Martinez defended his decisions by citing the situations in which Doolittle entered and their conversations before the game.

“Doolittle’s the closer. He’s the closer of this team,” Martinez said. “We’ve said that before, and this is based on conversations with Doo. If he’s available, as we talked about, then he’s going to pitch the ninth inning. He’s always been in the game when he said he was available to pitch.”

Doolittle looked fatigued the past few weeks, and the once-reliable anchor of an unreliable bullpen slowly became anything but. Three relievers combined for three scoreless innings Saturday before Doolittle departed with the team trailing. In his past 10 appearances, the closer has a 12.00 ERA.

“I’m still searching for answers, to be honest. I don’t know,” Doolittle said Saturday night after giving up four runs on three homers in the ninth in an eventual, 14-inning loss. “We keep trying to go back to the drawing board. We’re watching film. We’re looking at the metrics. I’m doing extra dry work before games. We’ve changed up a lot of my routine in the weight room, my maintenance and stuff. I just …”

Doolittle paused and shook his head.

“I don’t know,” he said. “It just wasn’t coming out tonight.”

On Sunday, Martinez underscored Doolittle’s importance to the team. The reliever the manager has trusted all season is in trouble, but he maintains his trust hasn’t wavered. Martinez pointed out the Nationals have “six weeks to go and then some.”

“We need him for the duration,” he said. “We need Doolittle to pull this off.”

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