If you throw high 90's with a wicked slider and a TBD quality change and you can hit the catchers glove, you will be a good starting pitcher. The constant study of the game and the players will help said player become and elite starting pitcher. Much in the fashion a guy like Verlander goes from being good to great after a few years in the league after really understand more about "pitching" than "throwing".
Bronson Arroyo can't just get away with "throwing" he has to "pitch". Chapman can just "throw" for the time being, provide value, and learn how to pitch later.
The main adjustment he needs to make to convert to a starter is endurance. And from what I hear he trains hard and is a freakish athlete.