The veteran right-hander pitched the sixth inning and allowed five runs while throwing 47 pitches in the frame.
"I threw a breaking ball," Witasick said. "It didn't feel right. I tried to throw another one -- I just let it go, and it didn't feel good at all. With the state of our bullpen, my fastball felt pretty good, so I had to stay out there as long as I could. Hopefully, just clear the inning. And it just swelled up. Like a golf ball in there. I guess we'll assess it tomorrow to see how we stand."
Witasick said that the injury occurred early in his outing.
"I can't pinpoint it," he said. "I remember throwing a breaking ball, and I thought mechanically I just did something wrong. You throw a breaking ball sometimes and it doesn't feel good. You throw another one and it feels good. I threw one, it didn't feel good. Came back, threw another one, can't pinpoint it. I'm like, 'This doesn't feel good.' "
Witasick signed with the Rays on June 12 after being released by the A's. He is 1-0 with a 6.15 ERA in 31 appearances this season for Oakland and Tampa Bay.
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.