Luiz was a big reason why Chelsea frustrated Bayern for so long in the CL final. He put it in a shift and was emmense defensively. While at times he does have mental lapses (big man syndrome) he always elevates his game on the big occasions.
The fact that at times at Chelsea he has looked better as a DM than a CB is testament to his versatility and quality on the ball.
I would argue that he at times struggled as a CB at Chelsea had more to do with their overall style. He is much more suited to play a high line style while Chelsea always drop deep.
Personally, I think Luiz would is a perfect fit. He is great in the air and is much more athletic than Masch. I think he complements both Bartra and Piqué nicely and could also be deployed as a defensive fullback.
I know a lot of you will disagree with me on Luiz which is fine. I just think he would do well here. Especially since Mou doesn't want him, I say why not?