Marouane Fellaini is definitely one of the better midfielders in the world. His power, grit, aerial ability, goal scoring and ball winning is precisely what Chelsea need in the midfield particularly if we're going with the nippy little inventive players. Personally I feel he presents us with so many options tactically that it would be extremely hard for teams to prepare for us. We could go with Mikel and Luiz holding with Hazard, Fellaini and Moses for games where power is most needed for instance. Oscar, Fellaini holding and Hazard, Mata and Moses/Marin when we need more inventiveness.... This is just off the top of my head but Fellaini is much, much more than a DM, what he provides to a team is less about position and more about presence.