Bumgarner underwent an MRI on Thursday after his early exit in the D-Backs' 7-6 loss to the New York Mets on Wednesday. He pitched just two innings in that game -- his shortest outing since August 2020 -- and allowed five earned runs on eight hits.
After the game, Bumgarner said he first experienced the discomfort in his left shoulder about three weeks ago, but he didn't inform the Diamondbacks because he thought the shoulder would heal quickly.
"This is just bad judgement on my part," Bumgarner said Wednesday. "I thought it was going to get better and go away sooner. The last three to four days, it's gotten worse and affected my pitching."
Bumgarner has posted a 4-5 record and 5.73 ERA over 14 appearances (12 starts) this season, his second in Arizona. He spent his first 11 MLB seasons with the San Francisco Giants.
The 31-year-old Bumgarner is a four-time All-Star selection and three-time World Series champion. He was named World Series MVP in 2014.
The D-Backs recalled right-hander Humberto Castellanos from Triple-A Reno in a corresponding roster move.