Webb missed his scheduled start Saturday against the Los Angeles Dodgers and the team said on its Web site he felt soreness Sunday while playing catch.
A recent MRI exam showed no structural problems, but the Diamondbacks' team physician, Michael Lee, diagnosed bursitis.
The 2006 National League Cy Young Award winner started against the Colorado Rockies last Monday and was shelled for six runs and six hits -- two homers -- with two walks in four innings while tying to pitch through the pain.
"It feels better than it did," Webb said. "If there is no structural damage, then there's nothing to worry about."
The move to the DL is retroactive to April 7, meaning he would miss only one more start before he would be due to come off the list.