Bringing back Zaheer means we are going backwards. I would rather bring in Varun Aaron.
If Zaheer is fit and in rhythm, he can add considerable teeth to the bowling attack. Ishant is anyways almost as mediocre as Zaheer on the field (if fielding is compared).
Its a pity that we dont have enough backups to account for injuries. And the backups we have are Mohit and Dhawal Kulkarni which does not show us in good light. And I am not putting money on Aaron for ODI's. World Cup is not a place to blood him.