After having a couple of setbacks with a right forearm strain this spring, the 25-year-old right-hander visited the noted arm surgeon on Wednesday and was told he needed to have the elbow-ligament-replacement procedure.
That procedure will take place at Andrews' facility in Pensacola, Fla., on Thursday. Recovery time is estimated at 12 months.
"Just wanted to say thank you to all the people who sent me their thoughts and prayers," Kohn said via his Twitter account. "I'll come back from this."
Kohn, a 13th-round Draft pick by the Angels in 2008, posted a 2.11 ERA in 24 relief appearances as a rookie in 2010, then struggled to a 7.30 ERA in 14 games in 2011 and wasn't called back up after being optioned to Triple-A in mid-July.
This spring, Kohn hurled five scoreless, one-inning appearances.