Collmenter, who was the team's Opening Day starter, has been struggling of late. Entering Saturday, he had not won a game since May 6 and had a 7.76 ERA over that span.
"I'm smart enough to know that something could have happened," Collmenter said. "Obviously I wasn't pitching the way I need to be to stay in the place that I want to be. Hopefully it's a chance to get some innings and get some things ironed out and get my command back, and that's really been the biggest component of my lack of success."
In order to make room for Webster, the D-backs designated J.C. Ramirez for assignment. Ramirez, who is out of Minor League options, was placed on waivers, and if he is claimed by another team, that team would have to keep him in the big leagues.
If he is not claimed, the D-backs can assign him to Triple-A Reno, and he can decide whether to accept the assignment or become a free agent.
Ramirez had a 4.11 ERA in 12 games for the D-backs, who would like to keep him.
"We hope he elects to stay with the organization because we know he'll be back and can help us," D-backs manager Chip Hale said.
• Left-hander Patrick Corbin, who is working his way back from Tommy John surgery, will make his next start for Double-A Mobile. Corbin's first rehab outing came last Monday for Class A Advanced Visalia.
• Enrique Burgos, who was placed on the disabled list last month with shoulder tendinitis, could start a rehab assignment with Mobile next week.
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.