This marks Axford's first stint on the disabled list as he begins his ninth season in the big leagues. While warming up Tuesday, Axford said it felt like his right arm couldn't extend normally when he released a pitch.
"You always pride yourself on being able to play in this game without going on the DL," Axford said. "I've always taken pride in knowing my body, understanding my body and working hard, diligently to make sure I can stay on the field. You deal with ups and downs and bumps and scrapes, and you get to know and understand what you can work through.
"I didn't think there was anything really going on. I threw one pitch, and it just kind of locked up back there and didn't feel good. It was something I never felt before. Threw another pitch after that, and it felt even worse. That's when I knew it was time to shut it down and stop."
If there's any silver lining, Axford said the MRI revealed no damage to his right rotator cuff and/or labrum. Manager Bob Melvin said there's no estimate on Axford's return, saying the organization will reassess the situation when he gets back on the mound.
Oakland promoted right-hander Jesse Hahn from Triple-A Nashville to fill Axford's spot on the roster. Though 37 of Hahn's 39 career MLB appearances are as a starter, the 27-year-old said he's ready to contribute out of the bullpen as a long reliever if necessary.
"It's an opportunity," Hahn said. "Anything I can do to help or be here, and help us win. Whether it's starting, in the bullpen, whatever it is, I'm just happy to be here. I'm going to out there with the same mindset and try to put zeroes up on the board."
Alex Espinoza is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.