Let's consider our cover at the moment;
Right back - Bosingwa is evidently first-choice, but Mancienne, Ferreira, Belletti and Ivanovic can all operate there, and Essien too in emergencies.
Centre-back - Terry and Alex are first-choice, with Carvalho with them - but injuries have limited him. However, we have cover in Ivanovic and Mancienne, and of course Essien who is so fucking awesome he can play anywhere.
Left-back - Ashley Cole is our first-choice LB, but we have both Ferreira and Bertrand as cover, and Bosingwa has proved he can play there as well.
So, we have Bosingwa - Alex - Terry - Cole as our first choice option. If Terry or Alex get injured, Carvalho or Ivanovic will most likely fill in. If Bosingwa becomes injured, Ivanovic may play there, or possibly Ferriera or Belletti is Ivanovic is already playing there. At left-back, if Cole becomes injured, we can put Bosingwa on the left and Ivanovic on the right, or simply just play Ferreira or Bertrand.
So, what I'm basically saying, is now Ivanovic is staying put, we DO NOT need any more defenders. Zhirkov may come as a LM and therefore offer cover at LB, but I really don't think we need to buy any more defenders. We have a really good mix of quality, experience and youth in every role.