Oh right, I said it, but you still implied it with your own post.
Just the exact same way Jones helps Martinez with attacking, a person with good defensive knowledge can come in and help with that, without apparently killing off everything Martinez has built with the attack. In fact, that's the same reason many managers across Europe have more than one designated assistant, or use their coaches more actively, so that you can have many opinions on how to improve certain aspects of play. I agree we need someone to help out Boberto defensively, but to say someone's going to come in and scythe everything Martinez wants from the team just because he wants to build a better defence is pretty silly, don't you think?