This might sound crazy but I think Mack might have a better chance at becoming better defensively than Duchene quicker than offensively.
He's still got issues, but I saw some things in the last game in Vegas against the Kings that showed me he approaches playing D better than Duchene does. He uses his body position better, and is more physical in trying to contain players. Duchene mainly just tries to shadow players and use his stick, but doesn't really "battle" that much defensively. I saw some battle in Nate defensively the last game. Definitely saw a change from last year where he looked a little lost.
It's still a learning process for him, and he's got to get used to thinking defensively when he's in the D zone, instead of just waiting to get the puck, but I saw a lot more patience in him. Wasn't cheating, and was willing to stay in position even if it meant he was the last forward to leave the zone in transition.
We all saw how quickly he learned the NHL game offensively last year, and I think he might have an equally quick learning curve defensively this year.