I think Luiz can make a difference in the attacks because he plays as a CB. If he is moved to midfield he will likely be dispossesed frequently. It is easier to get past attackers than midfielders.
He couldn't have closed him down because it was a two-on-one situation. Had he closed him down, the player would've simply made the pass and his team mate would've found himself unmarked and facing the goal. I actually think he made the right choice by trying to delay his decision till one of the midfielders returned to cover.
I really liked this article about Franks recent talk with the press after the win vs Bolton: http://thechelseablog.org/2012/02/28/lampard-save-the-battles-for-the-pitch/
He is, with no doubt, an excellent player. I just think our priority right now is a left winger so we can move Mata to the middle. Even Andre has admitted that we are conceding most of our goals from the right (our left).
I just can't understand how the referee can give the signal to take the free kick while the goalkeeper is obviously still setting up his wall. Then those diving cheats have the nerve to complain about the refereeing. Unbelievable!