Luiz is actually pretty good, he's just prone to having lapses in concentration and moments where he does some strange things. Those really stand out and get highlighted, which is understandable. He has pace, is strong in the air and is great with the ball at his feet though. In a defence that has been bad anyway over the last 18 months? That's a definite upgrade. Especially if 3 at the back works out, he can make up for Terry and Cahill being slow, their positioning can cover up for his mental lapses. Him being there can improve the whole defensive unit, if it's handled right. Conte seems like a manager who can make it work, I just wish he had pre season to do it and not 3 games into the season.