My take on it is that when he was in the rotation, he led the team or was tied in QS ICR. He was up there anyway. Some might find the QS a dinosaur stat, however, it does two things. It makes your pen better since now you don't have to expose shittier middle relief guys to get to your set up and closer guys, Two, if your starter is only allowing three runs, your team is still in the ball game.
I understand the injury and durability concerns. However, there are a lot of really good SPs out there who have been injured multiple times and stay as SPs. The only time he was a SP, he gave us a lot of 6 innings of quality pitching which is rare in this day and age even with some of big names.
He also did all those QS against really good teams in the AL East ballparks in a regular season where the hitting had already caught up to the pitching (I think his first start was late June/early July).
Hence, I say let him dictate if he can stay a starter or if he has to go to the pen with his performance that he has actually accomplished and not based on projections or suspicions.