"He just tried to put a little more into his throw and it just didn't feel good," manager Cecil Cooper said. "I don't think you can take any chances with Roy, he means too much."
Oswalt was forced to leave Monday night's start against Los Angeles with the sore hip. His start was originally pushed back from Saturday night to Tuesday. But now manager Cecil Cooper said right-hander Brandon Backe will start in Oswalt's place Tuesday in Pittsburgh.
Cooper said he couldn't speculate on when Oswalt will next pitch.
The right-handed Oswalt is 7-8 with a 4.60 ERA. He has at least 14 wins in six of his seven seasons, including back-to-back 20-win seasons in 2004 and 2005.