He has bone chips/spurs in his elbow, he can only go around 70-80 pitches an outing because of it.
Didn't Lester have this problem in 2016? He has a bone spur in his elbow but it isn’t a problem based on its size/location, iirc. It’s not rubbing on any ligaments/muscle/tendon/whatever. It could grow to the point of being a problem and he’d need surgery that isn’t overly serious.