Diaby is more of a box to box midfield player, and to be fair song was never really a DM, or he certainly never played like that. A fully fit diaby is better than song, there's no doubt about that, so yes he is a very good replacement, it just boils down to how long he can stay fit for.
Oh he was very talented and full of potential and obviously due to injuries, he has yet to live up to it. But I can't see how you'd take a fully fit Diaby over Song. Zlatan is right, Diaby is definitely better attacking but as an all rounder, Song >>> Diaby. It's not really up for debate to be honest mate. Diaby is such a liability defensively.
Actually it is up for debate, and a fully fit diaby, IMO is the better player, you can disagree if you like but it's not going to change my mind.
Song is as much as a liability defensively as Diaby, though. It's quite a strange one Song - Not very good at attacking but attacks a lot, very good at defending but doesn't chase back to do it. Diaby - Good at attacking but doesn't attack enough, not very good at defending but chases back adequately.
Song is in the first XI to make his league debut against Valencia, plays in place of Busquets alongside Xavi and Cesc in midfield.
There are many situations where he 'doubles' another Barca player, a bit positionally challenged/clueless still. Almost as if he doesn't billong there.
