I don't believe in the tooth fairly or Santa Claus (but strangely, I believe in the Easter Bunny) and I do not believe you can call up any 20-year-old who started pitching about 3 years ago and expect him to be Mark Prior, or even Mark Bruhele, out of the chute.
There are two other very young extremly talented pitchers in the Greinke mold who might be good barometers.
Like Greinke, Bonderman was rushed to the majors because his team stunk. He is only now starting to show the signs that he has the maturity to pitch effectively at the ML level. He probably should still be at AAA. But there isn't a scout out there who doesn't think he's a future #1.
Harden wasn't rushed quite as agressively, but there are those who say he could have used more time in AAA. Again, he's a consensus front-line starter.
I think if you look at these guys during their first year take the mid-point and you've got a pretty good idea of what to expect from Greinke.
I'd say: good first few outings as no one has seen his stuff (remember Harden), then ERA 4.25, WHIP 1.4, pretty much the same as Registered Guest above.