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Detwiler tried to throw off a bullpen mound this afternoon but cut the session short after about 25 pitches when his back stiffened up again.
"He's still not quite right," manager Davey Johnson said. "So he's going to have to wait a couple of days before trying to get back up on the mound."
The Nationals originally hoped to have Detwiler (who has been on the disabled list since July 4) back for Tuesday night's game against the Pirates, letting him slide into the No. 5 slot in their rotation coming out of the All-Star break. Once it became clear he wouldn't be ready in time for that, the Nationals adjusted their plan to have the left-hander start one of the games in Friday's doubleheader against the Mets.
But that option now is out of the question, according to Johnson.