If you think about it.. This is a great question! Correct me if I'm wrong, but in 2008 didn't we play Essien in centre back because we had an injury crisis? Well in the form Mikel's been this season, I don't see why, if we were to have a defensive crisis, Mikel couldn't play there. He's shown he's got the tackling ability to do it, his decision making has clearly improved and so has his first touch. We all know Mikel is now our #1 central midfielder (or defensive midfielder) which is the defender of the midfield quite literally.