Webb complained of shoulder stiffness during his Opening Day start last week, and he underwent an MRI on Friday -- results of which were not yet available when he met with the media prior to batting practice.
Arizona manager Bob Melvin said the MRI was more precautionary, and that he expected Webb to be ready to go next Friday in San Francisco.
"I think it's more just peace of mind than anything else," Melvin said of the test.
Webb received treatment from the team's trainers during Thursday's off-day and reported feeling better.
"I feel like we're getting somewhere with this now," Webb said.
"I know yesterday he felt a lot better," Melvin said. "He was in here getting some work down with our training staff. The stiffness was out of there, so hopefully that's progress."
Yusmeiro Petit will start in Webb's place Saturday.
"I haven't missed many starts in six years," said Webb, who has thrown over 200 innings in each of his full Major League seasons. "It's tough for me to sit out and watch."
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.