Webb began to experience discomfort in the shoulder toward the end of Spring Training and again during his Opening Day start. That was his lone appearance for the team this year.
Since then, he has undergone programs to try to strengthen the shoulder, and the initial diagnosis was that he had a strain of the teres major muscle, which is located outside the joint in the shoulder. That is less serious than if there was a problem within the joint, such as with the rotator cuff or labrum.
It was thought that rest and strengthening would take care of the issue, but each time Webb tried to take a step toward getting on the mound, it seemed he experienced discomfort.
The 2006 National League Cy Young Award winner was scheduled to throw his first bullpen session off a mound Friday, but while playing catch Thursday, his shoulder did not feel good, and the team immediately sent him to Phoenix for further examination.
"This guy has been battling for a long time to get back on the field," D-backs manager A.J. Hinch said. "So it's a pretty emotional time for him to have pain in his shoulder again."
Team physician Michael Lee is unavailable this weekend, so it is unlikely the team will know more until next week, and should Webb seek a second opinion, it could take even longer.
"There's some level of concern, but until we know, it's hard to predict," general manager Josh Byrnes said when asked about whether Webb could return this year.
The D-backs hold an option on Webb for the 2010 season at $8.5 million. Should they choose not to exercise it, they would instead pay a $2 million buyout.
Webb and the D-backs agreed on a three-year contract extension early last season, but the D-backs decided to withdraw their offer amid concerns raised over his shoulder when the team tried to get insurance for the contract.
When asked about the contract option for next year, Byrnes said, "Obviously we need to get our medical opinion formed and figure out a course of treatment for him, how it affects next year and beyond. That would be the case with anyone at this point."
Webb, 30, has been a mainstay in the Arizona rotation since 2003, when he finished third in the NL Rookie of the Year voting. In addition to winning the Cy Young in 2006, he finished second in both 2007 and 2008.
Last season, he was 22-7 with a 3.30 ERA, and Webb has been a workhorse for the D-backs. After being called up in April 2003, he made 28 starts and never made fewer than 33 starts in any season since.
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.