(Reed Saxon / AP) (Reed Saxon / AP)

Nationals left-hander Ross Detwiler received an MRI this afternoon to diagnose the full extent of his back injury and may have to miss his next start, scheduled for Monday in San Francisco.

Detwiler, who is 2-3 with a 2.76 ERA, left in the third inning of Wednesday’s 3-1 Nationals loss after he felt a spasm in the right side of his lower back. Detwiler had experienced small twinges in previous starts, but Wednesday the ailment prevented him from following through and extending to throw his pitches.

Manager Davey Johnson said he is “probably not that confident” that Detwiler will be able to start Monday. “He was moving a little better than he was last night. Hopefully he’ll be all right.”

If Detwiler cannot pitch, the Nationals would likely choose his replacement from either right-hander Craig Stammen or left-handed long reliever Zach Duke. Stammen may have the edge because he has been more effective this year, and because he pitched three scoreless innings Wednesday in relief of Detwiler and would be on schedule. In a versatile bullpen role, Stammen has a 2.25 ERA and 22 strikeouts over 20 innings.

The Nationals could choose Detwiler’s replacement from a shallow pool of candidates at Class AAA Syracuse, too, but only if the Nationals need to place Detwiler on the disabled list. “If we think he’s just going to miss one start, then we wouldn’t DL him,” Johnson said.

The Nationals signed veteran Chris Young to a minor league contract to serve as rotation insurance. In five starts at Syracuse, though, Young has punched up a 7.96 ERA and a 1.96 WHIP. Wednesday night, he allowed Columbus 11 hits and eight earned runs over 4 2/3 innings.

While the Nationals await word on Detwiler, they placed catcher Wilson Ramos on the 15-day disabled list. Ramos said his strained left hamstring feels the same as it did when he tore it earlier this season, which necessitated the minimum 15 days on the DL. Johnson said Ramos will need “at least two weeks, maybe three” before he returns.

Ramos will fly back to Washington to receive an MRI on his hamstring. He had felt great until last night and couldn’t pinpoint why he had strained the same hamstring twice. He posited that he had put too much pressure on his left leg as he recovered this winter from surgery to repair a torn right ACL and meniscus.

“Especially these two weeks, I was working a lot with my hamstring getting it more stronger,” Ramos said. “I don’t know what happened. I was feeling very good yesterday before the game. I don’t know. Maybe because I hurt the other leg when I got surgery last year, I put more pressure on this leg? Tried to keep the pressure even, but, unlucky.”