The move came after the Astros' seventh straight loss, 6-1 to Cincinnati.
Manager Cecil Cooper says Backe's role is undetermined for now. Cooper plans to stick with his current rotation for at least one more turn.
Backe has been sidelined since March with strained muscles in his left side.
The Astros optioned left-handed reliever Wesley Wright to Triple-A Round Rock. He was 1-1 with a 7.24 ERA in 19 games.