PHOENIX — Brandon Webb's pitching shoulder feels fine — until he pitches.

A nagging shoulder strain may sideline Arizona's ace for another month.

Webb, who hasn't pitched since opening day, will rest for three weeks after a setback on Friday. Then he hopes to ease back into the rotation with bullpen sessions and a possible rehabilitation start.

That means the Diamondbacks could be without the 2006 NL Cy Young Award winner until late May. Webb went on the 15-day disabled list April 13, retroactive to April 7.

"It's frustrating because what I feel right now, I feel fine," Webb said before the Diamondbacks hosted the San Francisco Giants on Saturday night. "Just when I throw is when I get the pain. Hopefully we'll just give it a little more time to heal and we'll be ready to go in a few more weeks.

"Obviously not real happy," Webb said. "Not what we wanted to hear."

Webb's prolonged absence could be a crushing blow for the Diamondbacks, whose hopes for contending in the NL West were based on stalwart starting pitching. Arizona was fourth in the division entering Saturday's games, 5½ back of the Los Angeles Dodgers.

Arizona manager Bob Melvin had hoped Webb, a workhorse throughout his career, would be back in the rotation by now.

"Obviously, he's our ace, and we certainly would like him to be pitching right now, but it's just something you have to deal with and move forward," Melvin said.

Melvin said right-hander Yusmeiro Petit would remain in the rotation until Webb returns. Petit is 0-1 with a 6.00 ERA in three appearances, two starts.

Webb's bullpen session was canceled Friday when the right-hander felt tightness in the shoulder while playing catch. It was supposed to be his second bullpen workout. He felt fine after his 33-pitch session on Tuesday.

The plan had been for Webb to throw to hitters if Friday's bullpen went well, then perhaps have him make a minor league start.

An MRI showed a small amount of fluid in the shoulder but no structural damage.

Webb gave up six runs in his April 6 start against Colorado. He was scratched from his next start before going on the DL for only the second time in seven major league seasons. The first was in 2003, his rookie year.

Webb has pitched more than 200 innings in each of his last five seasons.

Webb said he suffered the injury in his last spring training start, and he doesn't believe it is related to wear.

"I just think I did something in spring training that caused this," Webb said. "The wear and tear, on the (MRI) scans, it looks real good, so that's encouraging."