High three-quarters arm slot; effort in the delivery; does repeat delivery; very long arm action; hitters don't pick see the ball; arm speed doesn't allow for the arm to come through on time; affects the stuff and command; head whack; medium frame; stocky build; physically mature; lacks physical projection; highly competitive on the mound; can get frustrated when he's missing his spots; major league bloodlines.
Four-seam fastball; sat 95 mph early; has 96-97 in tank in put-away situations; dipped to 93-94 late in outing; fringe-average life; all arm side; hitters don't see the ball out of his arm action; fastball effectiveness matches grade; arm side control; below-average command; plus-plus arm strength plays; plus arm speed; but not premium arm speed to make up for long arm action.
Hammer curveball; nearly a 70-grade offering in the present; out pitch; solid-average command; spots it to both sides of plate; can freeze hitters by dropping it in at the knees; manipulates the offering; two-plane break; vicious vertical bite; sudden horizontal action.
Slider; shows below-average two-plane break; mostly horizontal action; got around it often in this look, leading to slurves or sweepers; hung over the plate; some tilt.
Changeup; threw it quite a bit; lacked the feel and touch; kept it down; firm; not much action; some arm side sink; high effort delivery didn't allow for deception and touch.
McCullers displays a four-pitch mix, but only two of the offerings project as solid-average or better. But since it is a 70-grade fastball-curveball combination, there is higher margin of error. The changeup doesn't need to be average, but it does need to be usable. As he continues to get a feel for his delivery, the changeup could reach that projection. Plus makeup and work ethic to get it there.
While I give him a chance to start, I'm skeptical that he will be able to hit his upside. A realistic role for him would be as a high leverage reliever in the backend of a bullpen, where he can pump his fastball from 95-97 mph with his lethal curveball. This is his realistic scenario because the life and command his offerings project as below-average; best I can see is 45-grade in both scenarios. The command won't be there due to the violent head whack and lengthy arm action, though it does allow for his stuff to be as good as it is.
The floor is that of a quality reliever. I think he could pitch in a bullpen right now, actually, because of how good the pure stuff is. But there is no need to pull the plug on the (unlikely) chance he could stick in a rotation this early in his development.