I'd be inclined to give Zab another season, unless he wants to go. No reason why he can't recover some form and physical conditioning, 31 is not that old.
He's two years younger than Sagna and a still a better attacking option IF he can regain his fitness.
I think much depends on how long his rehab period is. If its a case of 4 weeks rest and then a full pre season then I would take a chance on him. If the recovery period means he will miss a large part of pre season then it might be time to say farewell.