The cyst is just the latest in a series of arm troubles for Cain, who earlier in his career was a very durable pitcher.
After throwing 200 or more innings for six straight years from 2007-12, Cain hasn't reached 100 innings in either of the past two seasons.
The 31-year-old right-hander had surgery to remove bone chips in August 2014, then spent time on the disabled list with a tendon flexor strain in his right forearm last year.
Cain appeared in just 13 games -- 11 starts -- in 2015 and went 2-4 with a 5.