Gerrard, or at least the way he played under Houllier, needs a guy alongside him to win ball and cover back, as Gerrard likes to bomb forward.
Look at England at Euro 2004 where Gerrard was expected to play as a holding player and see what happened.
I like Hamman, but I'd prefer a more energetic and kinetic player in his place, a guy like Maniche or even Costinha.
Gerrard and his new midfield partner could share defensive and attacking duties, so you'd have a situation where you wouldn't be depending on a one particular player to do a particular job.
As Stu and Daglish rightfully said, when Arsenal miss one of their players the team continues to play its normal game and be effective, yet when Liverpool lost Hamman the whole "holding player in midfield" concept went out the window and unbalanced the team.