Wats biggest flaw was his speed ... MM is far quicker, though still not considered quick and still not able to play at his full speed yet. That will come with time and comfort.
Plus we remember Wat the Jr Sr ... Mack is a freshman 2/3 the way through his first year.
When people talk about adjusting to the speed of the game, it's not just the opposing speed they need to adjust to, it's also figuring out what speed they can play at. Right now for MM nothing is habit or reaction, he's still thinking, which makes him slow. With his physical build lack of extreme athleticism, he needs to either become a very good positional defender and/or develop some moves off the dribble.