If you want these tactics to work properly you need defensive players who are good in 1v1 situations and who can intercept key passes and close down opponents. David Luiz ticks all these boxes. Add in the mix the fact that he is going to be a starter for Brazil in a WC year and that he leaves the best League in the world to play in Ligue 1 and you'll understand why PSG had to break the bank for him.
Yes, he's a great man-marker and great at closing down the opposition, but he's not one who I can say intercepts key passes. His positioning and reading of the game isn't that good that he intercepts key passes. He's at his best when he has a solid defensive midfielder in front of him that reads the game well and fills in for him so that he can push up and win the ball. Plus, he's so good on the ball that he often starts off attacks and even acts as a pseudo-playmaker in the side. That's why I think he can be an excellent midfielder: he's great at winning the ball and in 1-on-1 situations. Plus, his technique is excellent, and his passing's great.
This is one of the reasons he does so well with Thiago Silva. Thiago Silva's a complete defender who reads the game excellently and is very aggressive in his defending. Plus, he has Gustavo and Paulinho to cover for him, both who read the game well and fill in for Luiz whenever he gets involved, defensively. In PSG, he has Matuidi, Motta, and Verratti who can read the game and cover him and the best defender in the world in Thiago Silva next to him.
I don't know why PSG are paying so much for him, still. 35 million pounds is the most I would pay for Luiz, IMO. 48-50 million is insane.