Essien is not a defensive midfielder. He doesn't play there for Ghana. He didn't play there for Lyon. Mikel and Makelele have often played behind him for Chelsea, and while he can be a DM, it is not his favoured position at all. Also, he had two poor seasons in succession now.
He's still better in that role than Cambiasso, even if it isn't his best postion, which is more box-to-box. How on earth has he had 2 bad seasons in a row? More ridiculous African hostility no doubt