UFC heavyweight champ Cain Velasquez -- the man who defeated Brock Lesnar -- is currently undergoing surgery to repair his damaged shoulder ... TMZ has learned.
UFC kingpin Dana White confirms ... Cain is at a Las Vegas hospital, where doctors are performing arthroscopic surgery on his right shoulder to repair a partially torn rotator cuff.
We're told Cain isn't sure exactly when the injury occurred ... but it may have happened during the Lesnar fight back in October.
No word on when Cain is expected to return to fighting shape.
UPDATE: Cain is out of surgery -- we're told it was "100 perfect .... only 3 small incisions."