The AL strikeout champion had warmed up in the bullpen without a problem, then watched the first half-inning from the bench. He threw three warmup pitches before leaving the mound and walking to the clubhouse for tests.
"I was warming up, feeling great," said Kazmir, who was 13-9 with a 3.48 ERA last season, when he led the AL with 239 strikeouts -- one fewer than major league leader Jake Peavy. "On my last warmup pitch ... I just kind of felt a little discomfort."
The Rays said the 24-year-old, who agreed to a $3,785,000, one-year contract last month, would undergo a MRI exam later in the day. His status was to be updated on Wednesday.
"When you have a guy like that, who's never really been hurt before, sometimes it's hard for them to gauge. We'll just wait," manager Joe Maddon said, adding that Kazmir's first spring training start, scheduled for Saturday, might be affected.
Maddon, who was talking to another player when Kazmir took the mound, wasn't sure what happened.
"He came out, just didn't feel right and said, 'that's it.' ... I didn't see it. I didn't see any of the throws. I didn't see any of the grimace. I didn't see any of that stuff. I just saw him walking off with his head down, so I knew something was bothering him."
Kazmir, an All-Star two years ago, made a club-record 34 starts last season and is being counted on to head an improved rotation also featuring two other promising young starters, James Shields and Matt Garza.
Kazmir, scheduled to work one inning, said he didn't feel a pop in his elbow.
"No, nothing like that. Discomfort," he said, adding that he shut himself down immediately because he didn't want to take any unnecessary chances.
"It was just a little scary because I never felt it before," Kazmir added. "It kind of feels like maybe I tried to do a little too much to get warmed up. Maybe I hyperextended it or something like that. That's kind of what it feels like."