PHOENIX — Arizona ace Brandon Webb is headed to the 15-day disabled list with tightness in his pitching shoulder.

The Diamondbacks decided to make the move, retroactive to Tuesday, after Webb tested his shoulder by playing catch on Sunday morning.

"I feel like we're making progress on it, but we just need a little more time to make sure it's totally good," Webb said.

The right-hander is confident the injury will heal quickly.

"There's no concern," he said. "The MRIs are all good. I mean, if there's no structural damage, there's nothing to worry about."

The club planned to make a corresponding roster move after Sunday's game against the Los Angeles Dodgers.

Arizona manager Bob Melvin said the Diamondbacks wanted to be cautious with Webb, who won the 2006 NL Cy Young Award and finished runner-up the last two years. Webb has been Arizona's workhorse since 2004, pitching at least 208 innings in each of the past five seasons.

"Early in the season, we can't be too careful with a guy like that," Melvin said.

Webb said he first started feeling tightness in the back of his shoulder during his last spring training start. He gave up six earned runs in four innings on opening day against Colorado and was scratched from his scheduled start Saturday.

This is Webb's second trip to the disabled list as a major leaguer. The first came in 2003, his rookie season, when Webb missed 15 days with right elbow tendinitis.

Yusmeiro Petit took Webb's turn on Saturday, and he's expected to remain in the rotation until Webb returns. Melvin said he's confident Webb will be back soon.

"If he's progressing with it, we have every reason to believe that the DL stint will take care of it," Melvin said.