Turnbow was 0-1 with a 15.63 ERA in eight games this season and gave up six runs in two-thirds of an inning at Chicago on Wednesday.
"His production had fallen to the point where it was hard to find spots to use him," Brewers manager Ned Yost said.
Yost said he'd rarely seen a career marked with such highs and lows.
"It's pretty unique," he said.
Turnbow had 39 saves in 2005 and 24 the following year, when he was picked for the NL All-Star team. But he suddenly lost his form midseason in 2006.
After converting 51 of 55 save chances at the start of his career, he went 0-6 with a 13.06 ERA from July 1 on in 2006 and lost his closer's job to Francisco Cordero. Turnbow was 4-5 with a 4.63 ERA as a setup man last year.
Milwaukee designated the 30-year-old right-hander for assignment and is responsible for the remainder of his $3.2 million salary this year. The Brewers have 10 days to trade him, place him on unconditional release waivers or send him outright to the minors.
Yost believes going to another team or to the minors will be helpful to Turnbow.
"A change of scenery will do him good," Yost said. "He needs to be somewhere where he can relax a little bit and take a deep breath and regain his confidence."
The decision wasn't easy, Yost said, but it was for the best.
"It's always difficult, especially with a guy that's been an All-Star for you," he said. "It's painful to watch him struggle. A lot of times you do it for his own benefit as well as yours and the organization's."
To take Turnbow's place, Milwaukee recalled outfielder Joe Dillon from Triple-A Nashville.