He already has the offspeed stuff, and it's plus.
From John Sickels:
The key, of course, is consistency. It's the same consistency every amateur pitcher is after. (And most of the pros, for that matter.)Greene already has 93-98 MPH fastball, the pitch earning 70 grades due to the combination of velocity and movement. He generates the heat with little effort and given his youth and build, even more velocity is possible as he matures. He already has a plus curveball and commands it better than most pitchers his age. His change-up is not as refined as the first two pitches but should be at least MLB average in time and very possibly plus.
With proper development he projects three plus pitches with impressive command, making him a top-of-the-rotation arm. Add in the intangibles and comparisons to Dwight Gooden or even Bob Gibson are not out of bounds if you want to dream and don’t believe in jinxes.
I suspect he'll find it sooner rather than later simply because of his plus-plus work ethic (also in the above scouting report) and plus control.
No one knows the future, but Greene's been described as a possible "generational player" by Perfect Game. Comparing him to Bailey and other HS pitchers who weren't considered as such is like comparing Scott Feldman to Trevor Bauer.