Of course both players in the pivot have the responsibility of defending but from what we've seen about the 4-2-3-1 there is always that one player in the -2- that has more freedom and a larger attacking responsibility. When Luiz and Mikel play together, naturally that's going to be Luiz as he offers more going forward. I've seen a few on here suggest that Luiz should replace Mikel as our primary (holding) DM in the pivot based on his recent performances in the pivot but he's plays a completely different role to Mikel. As @Leif just mentioned above, Luiz is doing a similar job as what Yaya Toure does for City. You can see Yaya at either ends of the pitch but that doesn't mean he's neglecting his defensive responsibility - he's just been given the freedom and is probably encouraged by Mancini to join in on the attacking phase of play. I suspect it's the same with Luiz. Essentially, Mikel could be our Barry, and Luiz our Toure. Your Lampard analogy is quite relevant but do remember that we did set up a defensive 4-2-3-1, not an expansive one as we see today. In an expansive style of football - there will always be gaps for the opposition to exploit. It's not so much about making sure that everyone is perfectly in position... That's what you do when you're a defensive minded team. What CFC needs to learn is how to defend with the ball. It's our biggest flaw IMO.