As a centre-half Luiz was infamously described by Gary Neville as playing like he was “controlled by a ten year old on playstation." A bit harsh maybe but you could see his point.
Playing at centre-back Luiz has a tendency to over complicate matters. He looks to dribble past players and overload into midfield. Whilst making his performances very watchable they also tend to lead to defensive mistakes.
As a defensive midfielder however, Luiz looks like a natural. His attacking flair can be more easily utilised and when it goes wrong it doesn't cause the defensive chaos like it can when he's at centre back. Defensively as well, Luiz is skilled enough to read the game.
Despite being initially resistant to playing Luiz as a defensive midfielder, Mourinho has come to use him there more and more playing him there recently against Liverpool and