"I don't think I made the existing injury worse," said Withrow, the club's 2007 first-round Draft pick, who is making a transition from starting pitcher to reliever.
Withrow, who with his wife survived an offseason rollover auto accident with only minor injuries, said this is the first time he's been injured while pitching. His earlier episodes were triggered by conditioning exercises and weight lifting.
"This is new for me, which is why I think it's not as bad," he said. "I'll take it easy a couple of days and make sure it doesn't flare up."