You thought wrong. He was drafted based on his size, athleticism and potential. He was not a dominant rebounder or defender/shot blocker in college. He's just as raw, probably more raw, on defense as offense. It looks like no one has ever tried to teach him basic defensive fundamentals - that he skated by based on his size and athleticism. In the NBA, he's playing against guys just as big and athletic, but more skilled. He definitely needs a LOT of work on his basic defensive skills. At least on offense he can dunk and has a soft touch on his jumper. He needs a lot of work on both ends of the court.
The good news is he has the size, athleticism and basic skills to be above average at both ends. The bad news is it will take him a while to get there - if ever. I think he will get there, but it's going to take about three years before he will become an above average NBA starting center. The additional good news is he will only be 23 at that time. Patience everyone, he's a work in progress.
BNM