Katherine Frey / The Washington Post Katherine Frey / The Washington Post

Nationals left-handed reliever Zach Duke will replace Ross Detwiler on Monday in San Francisco, becoming the first rotation replacement the Nationals needed this season.

The Nationals are awaiting results of a third MRI Detwiler underwent this afternoon on the right side of his back. Detwiler left his start Wednesday in Los Angeles with a slight oblique strain, and he may or may not land on the disabled list. Because the Nationals have an off day Thursday, they could stay on rotation for four extra days before deciding whether or not to put Detwiler on the DL.

Duke, 30, has made 168 career starts and was named an all-star as a starting pitcher in 2009. But he last started a major league game on July 10, 2011. After he went 15-5 as a starter last year with Class AAA Syracuse, Duke signed with the Nationals this winter knowing he would serve as a long reliever, with a chance to fill in the rotation in case of injury.

“Very excited,” Duke said. “I just want to give the team a chance to win. That’s unfortunate, Ross’s situation, but I’m going to make the most of the opportunity.”

Duke has struggled in his transition to full-time long reliever, receiving few chances and not pitching with much effectiveness when given the chance. He has an 8.40 ERA, allowing 22 hits and four walks in 15 innings.

“I’ve always just said I’m a pitcher,” Duke said. “It’s my job to get to people out. This is a little more usual for me. It’s one of those things where, I’m not going to get used to it.”

Duke said he will not be able to fall back into his old routine. He threw a bullpen session this afternoon, two days before his start rather than the customary three. He also won’t lift weights and run on his usual schedule. “I’ll just have to take where I’m at and take whatever I’ve got out there,” Duke said.

Duke received the nod to replace Detwiler over Craig Stammen, who has a 2.25 ERA with 22 strikeouts and 20 innings. In Manager Davey Johnson’s view, Stammen has been too valuable as a versatile, multiple-innings reliever to remove from his role.

“Stammen’s been very good, and I don’t really want to change him out of the role he’s in,” Stammen said. “Duke, I haven’t really been able to get him work to keep him sharp. This will help get him back where he needs to be. It kills two birds with one stone.”

Duke starting will also leave no left-handed pitchers in the Nationals’ bullpen. Johnson has not used Duke in anything even close to a high-leverage situation, anyway, so that shouldn’t make a major impact.