His on-ball defense has looked pretty good at times. Obviously Kyrie made him look silly a couple of times last night, but Kyrie does that to everybody. Other aspects of his defense are lacking. He tends to ball watch and lose track of his man.
I've noticed that too. He needs to learn that the guy he's assigned to defend is still his assignment whether he has the ball or not. He's young, he'll learn that in the NBA, if you lose contact for a split second, they'll make you pay.