D-backs manager Chip Hale explained the decision by saying that Collmenter has struggled with command and velocity of late and that a move to the bullpen might help him. Collmenter was a valuable reliever for the D-backs in 2012 and '13 before being shifted back to the rotation last season.
A lack of innings pitched by the rotation this season has taxed the D-backs' bullpen, and the team has had to carry an extra reliever, leaving the bench short-handed.
"For us as a group, we think we can get back to 12 pitchers at some point because he covers us so well in the bullpen," Hale said of Collmenter.
Webster was acquired from the Red Sox this offseason in the deal that sent Wade Miley to Boston. The right-hander struggled during Spring Training and opened the season with Triple-A Reno.
After being roughed up in his first two starts, Webster was placed on the disabled list with shoulder fatigue and remained there for more than a month. Following his activation, he pitched well in his first two starts before getting hit hard in his last start.
Webster is 1-2 with a 9.00 ERA for Reno.
"He's got good stuff," Hale said. "He's had mixed results in Reno, but they like the way he's throwing the ball right now, and we're going to give these guys a shot. Now it's time to see what we have."
Steve Gilbert is a reporter for MLB.com. Read his blog, Inside the D-backs, and follow him on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.