Detwiler has been on the disabled list since July 4 with a back strain. He played catch on flat ground on Thursday, but continues to have soreness in the back the day after throwing.
Johnson doesn't expect the left-hander to make his next start, or the one after that against the Mets next weekend. Rookie Taylor Jordan will continue to fill in for Detwiler in the Nationals' rotation.
"For me, [Detwiler] would have to throw today or tomorrow, and do a side [session] and then get out and pitch in a game. And I don't think we're going to do it by [next weekend]," Johnson said. "But I'm going to put that question to our great medical staff, see if they can't get him ready to go."
If Detwiler is unable to start in the Nationals' doubleheader against the Mets, Johnson said that he would probably start Jordan and Ross Ohlendorf rather than dip into the Minor Leagues for an additional starter.
Tom Schad is an associate reporter for MLB.com. Follow him on Twitter @Tom_Schad. This story was not subject to the approval of Major League Baseball or its clubs.