Zach took a big leap forward in his defensive play in the year after his draft year. He might be able to take another step. I think there's room for growth, mentally and physically.
We shouldn't expect 21 year old D to be very good defensively. A lot of it is just experience. And he should have more motivation to learn now that he knows his minutes aren't guaranteed. He's never had his minutes threatened before, not in the NHL.
I'm wondering if he can work on core strength and quickness more next offseason. (I remember saying this a year ago, and he doesn't look any better, perhaps the arm in a sling was an issue). He's a big muscly guy, with powerful strides. But the quick small movements to adjust angles is what he needs, and the quick down and up off the ice too. You can't get those with upper body workouts.