Alex Brandon/AP Alex Brandon/AP

Ross Detwiler will miss at least one start with a slight oblique strain, a diagnosis the Nationals discovered after he underwent a second MRI exam this afternoon. The severity of Detwiler’s injury, Manager Davey Johnson said, will determine who takes his place in the Nationals rotation and where his replacement comes from.

Johnson said Detwiler hopes he will only need to miss one start. If that is the case, the Nationals would choose between Zach Duke and Craig Stammen to make Detwiler’s start Monday in San Francisco. In that scenario, according to one person familiar with Johnson’s thinking, Duke would be the likely choice. Johnson views Stammen as perhaps too valuable in his current role – a versatile long man who can protect leads – to remove him from it.

If Detwiler lands on the disabled list and the Nationals need a long-term fill-in, they would dip into a shallow stockpile at Class AAA Syracuse. The Nationals signed veteran Chris Young as an insurance policy, but he has a 7.96 ERA through five starts and, because of his late start in spring training and intermittent schedule to begin the season, the Nationals do not view him as ready to pitch in the majors.

The choice would come down to Yunesky Maya, the Cuban defector who is on the 40-man roster by virtue of the ill-fated four-year, $6 million deal he signed in 2010, or Ross Ohlendorf, a veteran who joined the Nationals on a minor league deal this past winter.

At Class AAA Syracuse, Maya is 1-4 with a 5.07 ERA, 34 strikeouts and 17 walks in 49 2/3 innings. Ohlendorf is 3-4 with a 4.96 ERA, 37 strikeouts and 19 walks in 49 innings. Maya has made 10 career big league starts and last pitched in the majors in 2011. Ohlendorf has pitched in the majors in parts of six seasons. In 2012 and 2011, a span of 18 starts and 87 1/3 innings, Ohlendorf punched up a 7.94 ERA.

Maya started in Syracuse on Thursday night, and so the Nationals would likely have to start either Stammen or Duke on Monday even if they wanted to put Maya into their rotation for multiple starts.

Both Duke and Stammen welcomed the idea of making a spot start. Duke had been a starter his entire career before September last year. Stammen was in the Nationals’ rotation in 2009 and 2010 and pitched as a starter until last year.

In 35 appearances (including 19 starts) in 2010, Stammen went 4-4 with a 5.13 ERA over 128 innings. He has since become an often-dominant reliever, striking out 9.9 batters per nine innings this season.

“I think I would pretty much try to do exactly what I’m doing in the bullpen,” Stammen said. “What’s kind of changed, from the beginning part of my career to now, is I understand what the mental challenge of the game is, and I’ve learned how to control that. I think if I can apply that, no matter what inning I pitch, I feel like I’m going to be successful. Whether it’s starting or relieving, I’m going to still have the same thought process.

“If I ever got to start again, I would be interested to see how I would do. I feel like I could do better. I’m older, more experienced. I feel like my stuff is better. I feel like I’m better than I was then.”

Duke has had difficulty making the transition from starting to relieving. In only nine appearances this year, Duke has an 8.40 ERA. As a starter last year at Syracuse, Duke went 15-5. “I’m comfortable there,” he said.

Johnson said he hoped to make a decision by Saturday in order to give the new starter an opportunity to prepare.