Wang had his worst outing of the season Tuesday night, lasting just 3 1-3 innings and giving up five runs on seven hits and three walks in a 5-4 loss to the Tampa Bay Rays. His record fell to 2-3 and his ERA rose to 6.10.
"Chien-Ming is very valuable to us and I feel like in his (four) starts that he hasn't gotten better with his mechanics," Johnson said prior to Wednesday night's game against the Rays.
"I still like him as a starter. I like his arm strength, I like his stuff," he added. "He hasn't seemed to get his rhythm back, so hopefully he'll do it, because I'm definitely going to need him."
Wang began the season on the disabled list after injuring his left hamstring in spring training. He hasn't pitched in more than 15 games in a season since 2007.
Detwiler was 3-3 with 3.88 ERA in nine starts before moving to the bullpen. He is scheduled to start Sunday against the Baltimore Orioles.
After relieving Wang on Tuesday night, Detwiler pitched 3 2-3 scoreless innings, retiring nine straight hitters and 11 of the 12 he faced.
"He was dominating last night, very aggressive after the hitters," Johnson said. "In his last two starts, he was pitching defensively, not attacking the hitters. So I think the stint in the bullpen, if anything, helped him."