Webb missed the team's trip to New York and instead will visit Dr. Keith Meister of the Texas Rangers in Dallas, Diamondbacks general manager Josh Byrnes said Thursday.
Webb has consulted numerous doctors but has been unable to relieve stiffness and soreness in the shoulder. The 2006 NL Cy Young Award winner has not pitched since opening day and has been limited to throwing off flat ground.
"He's not probably making the progress we were hoping for, so I think we have to make an assessment about what to do next," Byrnes told reporters in a conference call. "He's worked hard. It's just when it comes to throwing, it just hasn't been much different recently than it was going back to late May or June."
Byrnes declined to address the possibility that Webb might require surgery, saying he wanted to be cautious.
Earlier this month, after meeting with team doctors and three other physicians, Webb rejected surgery and opted for a more conservative approach. At the time, he expressed hope that he would pitch again this season.
That appears highly unlikely after Thursday's announcement. Now, the question becomes whether the Diamondbacks will exercise their option on Webb for 2010. The deadline is shortly after the end of the World Series.
"We've still got in the neighborhood of 100 days, I guess, before we have to make that decision," Byrnes said. "He's been a tremendous pitcher for this organization and a tremendous person within the walls of the organization, so we're going to have to gather all the information we can before making the decision."
Webb was the Diamondbacks' eighth-round selection in the 2000 draft.
The 30-year-old Webb has built a reputation as a front-of-the-rotation workhorse. He pitched more than 200 innings every season from 2004-2008 and won 56 games between 2006 and 2008, leading the Diamondbacks to the NL West title in 2007.
He won the NL Cy Young Award in 2006 and finished second each of the last two seasons.
Until this season, Webb had only made one trip to the DL as a major leaguer, missing 15 days with right elbow tendinitis in 2003.