Too me, the age decides it. He is 45, 5 years away from 50, which is pretty close.
-He already set for 2000, strikeouts. He has something like 2018.
- If he doesnt win 200 this year, I would play another year because your could also get 600 Games, 3000 Hits Allowed (not saying he wants that one)
So if you look at it, he needs some more milestones that need to be finished, so even if he is not on the Red Sox, I say another year will fit him good. Since is major league debut (2 ERA) he has been always in the 4 ERA.