St. Louis Cardinals pitcher Alex Reyes has a partial tear of the ulnar collateral ligament in his elbow and will get a second opinion, but season-ending Tommy John surgery is likely, a source told ESPN's Jim Bowden.

Reyes, a leading contender for National League Rookie of the Year, was scheduled to undergo an MRI on Tuesday afternoon. General manager John Mozeliak said the Cardinals will wait until Wednesday morning before announcing the next step.

"I'm concerned about Reyes. Any time you send someone to get an MRI, it's concerning. I'm not going to speculate what the MRI is going to say. We'll save that for tomorrow," Mozeliak said Tuesday.

"I've had experiences where we've speculated the person was going to require Tommy John and didn't. We've felt at other times they were going to be fine and needed Tommy John, so here we are."

Reyes, a 22-year-old right-hander, made his major league debut Aug. 9 and was 4-1 with a 1.57 ERA in five starts and seven relief appearances.

Reyes, whose fastball touches 100 mph, is competing with Michael Wacha to be the team's fifth starter. He also had been selected to pitch for the Dominican Republic in the World Baseball Classic.

"I don't like MRIs," manager Mike Matheny said. "I'd rather he wasn't getting that right now."

Heading into spring training, there had been speculation that the Cardinals might lessen Wacha's workload by moving him to the bullpen, but Mozeliak said he expects a healthy Wacha to be in the starting rotation. He's 32-21 in his career for the Cardinals.

"When right, he's been very good," Mozeliak said of Wacha.

Reyes first experienced soreness in his elbow late last week and alerted the team's trainers, who decided to wait until Tuesday's physical to administer tests.

Mozeliak said an MRI taken of Reyes' elbow in 2013 showed a strain. He didn't begin his minor league season until June that season.

ESPN's Keith Law ranked Reyes the No. 2 pitching prospect in baseball.

ESPN's Mark Saxon and The Associated Press contributed to this report.