Detwiler, the Nationals' first-round pick in the 2007 First-Year Player Draft, had surgery in February to repair a labral tear in his right hip. He missed Spring Training and has not pitched for Washington this season.
He pitched for the first time in a rehab start on June 10 for Class A Advanced Potomac. He tossed three shutout innings with two strikeouts. In three rehab starts with Potomac and Double-A Harrisburg, Detwiler had a 0.84 ERA in 10 2/3 innings.
"He's going to need a little bit more time in the Minor Leagues," Nationals general manager Mike Rizzo said. "We want to see him get a little more arm strength and get his velocity up to where it needs to be for him to compete successfully up here. Once he's ready, we'll certainly consider him a guy to come up."