Wang has made one relief appearance this season after missing nearly all of the first two months because of a strained hamstring. The 32-year-old allowed one run in three innings Friday at Atlanta. He is scheduled to start Wednesday against the Miami Marlins.
Detwiler's season got off to an excellent start, as the 26-year-old left-hander went 3-1 with a 1.59 ERA over his first five games. But the Nats' 2007 first-rounder has tumbled since, going 0-2 with a 7.08 ERA his last four starts.
"It's a tough decision, and I think it's tough for the whole ballclub," Johnson told reporters.
Johnson explained the move was made in part because Wang needed to pitch less often in relief as the right-hander would've been treated like a starter in the bullpen. Detwiler is more versatile in that regard.
"It would be very hard for me to handle a (former) 19-game winner, who made a great comeback from arm surgery, to have him in the bullpen," Johnson told reporters. "Here is a guy who has been in pennant fights, too, and knows what the pressure is all about. So it's a good opportunity for him and for us, to have him be in the role he is accustomed to, and that's to start. It doesn't lessen the fact that I think Detwiler is one heck of a pitcher."