PHOENIX -- After having to leave Thursday's start after just two innings, Brandon Backe was placed on the 15-day disabled list with a sprained ulnar collateral ligament in his right elbow. He underwent an MRI on Friday afternoon which confirmed a moderate sprain, and he will be out at least six weeks.

To take Backe's place on the roster, the Astros recalled right-hander Ezequiel Astacio from Triple-A Round Rock.

Astacio, 26, pitched six scoreless innings in his only outing of the season for Round Rock, allowing four hits while walking one and striking out four on Sunday at New Orleans. He appeared in 22 games -- 14 starts -- for the Astros in 2005, finishing 3-6 with a 5.67 ERA.

Although Astacio has been groomed as a starter, he'll work in relief for now. Rookie Fernando Nieve will take Backe's place in the rotation.

"[Starting is] what Nieve would prefer to do. It's time to go out there and strut your stuff," said manager Phil Garner. "We've got some guys wanting that opportunity. You've pitched good enough to get here. You got your foot in the door and don't let anyone else in that room with you."