Whether "rushing" players stunts growth is an issue many forum users disagree on. Most, but far from all, think that if the player isn't playing "above his head" talent-wise and is seeing regular playing time, then he will not be harmed from a development standpoint.
That said, is *is* a big deal because a large part of this game is finance management, and (if you put him in the ML now) by the time you're ready to win at the ML level, the stud pitcher is going to be demanding the big bucks, making it harder for you to surround him with the talent to win.