Ivanovic has been doing great at RB here. No need to change that. Maybe he also wants to give 20 year old Zouma a chance. He has told him (Zouma told the media) that he will put him on in difficult circumstances to test him, like he did with Varane at Madrid. Also, Luis is a LB and I'm not sure he would fit well in the CB position. Zouma is a CB by trade, and a good one at that.