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Ross Detwiler threw a bullpen session on Wednesday afternoon at Nationals Park with pitching coach Steve McCatty and General Manager Mike Rizzo watching. He threw well and felt fine, according to both Rizzo and Manager Davey Johnson. But the true test of his improvement from an oblique strain will be if Detwiler doesn’t feel any discomfort Thursday.

“We just have to make sure he isn’t having any discomfort in his side,” Johnson said.

The last time Detwiler threw a bullpen session was May 25 but the following day he felt discomfort in his side while fielding ground balls from the mound. He then landed on the disabled list, retroactive to May 15, his previous start. At first, Detwiler was expected to miss only one start but rookie Nate Karns was called up from Class AA Harrisburg and has made two starts in Detwiler’s place. Karns will start on Friday against the Minnesota Twins in Washington.

If Detwiler doesn’t suffer any setbacks on Thursday, the left-hander is expected to make a rehab start next week and throw 40 to 50 pitches,  perhaps Monday. Johnson said Detwiler would likely only need one rehab start.

Before his injury, Detwiler had been one of the Nationals’ best starters. The 27-year-old has a 2.76 ERA over 45 2/3 innings with 23 strikeouts and 10 walks. He appeared to pitch a lengthy up and down bullpen session on Wednesday, a way to test the injury to his side. “Threw the heck out of the ball,” Johnson said.