Oswalt scratched just before gametime
The announcement was made two minutes before the game was scheduled to start and the game's start was pushed back to 2:15 p.m. ET. Fernando Nieve was scheduled to start in his place.
Oswalt left his last start May 29 with a tight hamstring, but had recovered from that injury and was in the lineup Sunday.
Oswalt said his back began bothering him two days ago and that it worsened when he was warming up before the game. He said he threw about 20 pitches when he realized he wouldn't be able to play.
"It felt like somebody hit me in the back," he said. "I've never had back problems so I don't know what to expect with this."
Astros manager Phil Garner said he doesn't expect Oswalt to go on the disabled list, but that team doctors will be monitoring his condition.
"I've just got to get it well and we'll go from there," Oswalt said.
Copyright 2006 by The Associated Press