Well I only saw Essien a few times at Real Madrid, but as far as I remember in the many years of watching him he was never a very good DM anyway. He could do a job there because (in his prime) he was the ultimate utility player that had the pace and power to play just about anywhere. I don't think making him full time DM is a wise move, but he can be backup there.
Either Mikel will stay, Luiz will become a full time DM, Romeu will get his season to shine or we'll sign the likes of De Rossi. Ramires&Essien are not DMs but both can sort of do a shoddy job there if necessary (neither has particularly good passing either - both much worse than Mikel's even if he has become very conservative with it in the last couple of seasons).