"For a guy like him, this isn't something you can mess around with," manager Bob Melvin said before the Diamondbacks played Colorado on Wednesday. "I'd hate for him this early in the season to go out there and pitch with some soreness already, or some stiffness."
Webb, whose 22 victories led the National League last season, experienced stiffness in his shoulder in the fourth inning of Arizona's 9-8 Opening Day victory Monday over the Rockies.
The 2006 NL Cy Young winner blamed problems with his arm angle, resulting in a diminished sinking fastball, for his poor outing. The right-hander allowed six runs on six hits, including two home runs, in four innings.
"He played catch yesterday and was OK," Melvin said, "then came in today and it was bothering him a little bit, so I'm going to try to be proactive on this one."
No medical tests are planned as yet.
"I think we'll wait and see how he feels tomorrow," Melvin said.
Webb will not throw at all for two days so the training staff can "just work on him a little bit and try to get whatever is in there out of there," Melvin said. "We'll take it day by day. He'll play catch one day before he throws a bullpen."
Petit was 3-5 with a 4.31 ERA in 19 games for Arizona last season, eight of them starts.