Underestimating them is dangerous. Exactly the type of match for someone like Luiz to have a freak performance.
I am not underestimating Chelsea at all ... not one of those who think Napoli will crap them out today.
Still, Chelsea needs to play a closed game at the back to have a chance vs this Napoli team - playing high defense as they often do in EPL would be suicidal, imo - and their best defense to give them such a game, with the personnel they have would be Bosingwa - Ivanovic - Cahil - Luiz ... Luiz can play well at LB - solid in defense, good going forward, while he lacks the tactical awareness to be a top CB just yet.
Anyways, Chelsea can win this game if they play good, tactical football and don't bare themselves in the back with a high defensive line ... but not sure AVB won't end up doing just that.