It's about what the body is conditioned to handle. Wainwright, prior to pitching as a reliever his rookie year, had thrown 160+ IP in the minors three times.
"Let the young man throw the ball" is simply not a blueprint the Twins can follow with Graterol. They need to be extremely thoughtful and careful about his usage and strain level. This is a vital asset to their future, no matter his role.
So how is he ever going to be stretched out to become a starter if they don't start now?
If he's dominant out of the bullpen this year, are you really going to send him back to the minors for two years so that he can build up his innings to start again?
I'm not buying the argument that he can always go back to starting later.