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

Nationals left-hander Ross Detwiler will receive a second MRI on his ailing back Friday after his exam Thursday afternoon “inconclusive,” General Manager Mike Rizzo said. The second test, Rizzo said, will be broader, covering a wider area of his back.

The first test showed an “irregularity” in the right side of Detwiler’s back, Manager Davey Johnson said.

“I don’t know until tomorrow,” Detwiler said.

Detwiler (2-3, 2.76) left his start Wednesday night after he experienced back spasms while running to first after a bunt and then covering first base. Johnson said earlier Thursday he expected Detwiler to miss at least one start, with Craig Stammen or Zach Duke the two likeliest candidates to replace him.

Meanwhile, Rizzo expected Jayson Werth’s slight tightness in his hamstring to keep him on the disabled list longer than expected. Werth had planned on joining the Nationals on Saturday, the first he was eligible. Instead, Rizzo said, Werth will rehab more in Potomac before coming west and coming off the DL.

“We want to see where he’s at after he plays a couple games,” Rizzo said. “It’s safe to say he’s going to be pushed back a couple days.”