Hopefully he pitches mostly in the minors, no reason at all to rush a guy his age. If he does pitch alot this year in the bigs, who knows what his numbers will be? Rookies are darn near impossible to predict. He could start of flaming hot 2.5 ERA 1.10 WHIP or he could just as easily (and more likely) start off very shaky, 4.5+ ERA 1.40+ WHIP. Don't bank on every young player, pitcher or hitter, to perform like the Liriano, Verlanders, Pujols' of the world.
In other words if its a redraft league he probably shouldn't even be drafted. If it looks like he will get significant playing time this year he should still just be a low round, low risk/high reward type of pick for you.