It has to be Alonso. As much as i hate to say this but Mascherano as well. He has done a hell of dirty job covering our back 4.
Mascherano's a great player and his style complimented Xabi but whereas the team could function almost as well without Mascherano (so long as a similar 'physical' type of player was playing alongside Alonso - Sissoko), it couldn't without Alonso - as we all saw laast season
He was the pivotal player to the way we used to play and i find it staggering that rafa stuck with the same formation once Alonso had gone as without him it just didn't function anywhere near as effective
another dud decision was effectively swapping Anelka for Diouf
It's a no brainer as to which of the two is the better player - far better
