I will take a shot. First of all to me, Luiz at CB > Luiz at CM. However, current Chelsea squad with Luiz at CM > current Chelsea squad with Luiz at CB. So what I am saying is, Luiz is a better defender than he is a midfielder, but at the same time, he is Chelsea's best midfielder of the present options. Lampard has the passing range, footballing brain but what he lacks is speed and defensive workrate. Mikel on the other hand, passing range is very limited, workrate is very poor and is very ponderous on the ball. Luiz has the tendency to be rash and also tries to do too much sometimes that can end up costing us. But with that, you get a lot of upside. We have seen him play lovely long balls many times this season, he is always looking to make something out of our possession rather than just wait and ponder over the ball while the opposition gets back into shape (ala Mikel).