With the clear-cut plus fastball from the left side, the flashes of the plus hook, and developing changeup, Duffy has the type of stuff you put near the top of a big league rotation. Whether he ever develops the type of command in the zone to be that type of pitcher remains to be seen. Right now, he's not a guy who is pinpointing his pitches and relies more on challenging hitters with his plus stuff over the heart of the plate.
Right now, his overall command of his secondary pitches make him more of a third starter or perhaps a two. But, if he can consistently spot his curveball or changeup, then we suddenly have an ace-type pitcher on our hands. In either case, the Royals have to be very pleased that they have Duffy back on the baseball field.