Every time he sees the ball, he just charges at it at 100mph, with no regard given to whether it is appropriate to make a challenge.
This is true and this makes him the worst defender i've ever seen in Juve, and i've seen lots of them.
I am not exaggerating. You don't even need to follow football to realize that what he's doing is wrong. I'd understand if the ball was red, because he's as stupid as a bull, but the ball is not red and he's still running full speed at it, no matter where the ball is. His timing is awful. Actually, he has absolutely no sense for it.
Yeah, when it works, it's good for the eye. With his speed he can get first to the ball very often. But as a defender he needs to minimize his bad decisions and since his decision is to always run at full speed to get the ball, he'll be late very often (because he decides to run for the ball even in situations when there is 0% chance to get it) and even if he's late only 3 times out of 10, those three times will cost his team dearly. Either he'll leave all the space in the world for the opponent (he can't control himself when he runs so fast, so he can't stop and run back), or he'll catch opponent's leg and get a red card.
What's amazing is that he keeps doing it. There is absolutely no chance that the coaches he worked with didn't tell him that what he's doing is very wrong and that they didn't try to teach him how to defend. They surely told him all this.
So, since he keeps doing it, the only conclusion is that he's extremely stupid.