Well it isn't because of one player, Chelsea have done a whole change.
Drogba, Anelka, Kalou, Malouda, Essien, Ballack, Bosingwa, even Miereles are all gone.
Torres, Hazard, Mata, Oscar, Ramires, MArin, Moses are all part of the new forward idea of Chelsea. The defensive line of Chelsea is functionally the same but has gotten younger.
I am not shocked at Chelsea's success. Chelsea have quality depth. This is what creates success in all competitions which is the goal of top clubs. Bertrand is in for Cole and Cahill is in for Terry.
Chelsea are a money club. Like at Real Madrid, Mourinho came in and a bunch of players were let go and a bunch came in. Look at Real's success. Chelsea has done the same thing.
In terms of tactics, Chelsea have lost some skills and gained others. Chelsea aren't aerial any more or wide. Cahill and Ivanovic will get headers and Moses will get wide but for the most part Chelsea is not a ground ball team a cutting team. What Chelsea have to learn is how to cut quicker without the ball and space better when high pressing. but, they have so many quality players they can apply this towards many different opponents.
The Champions League win for the senior players was vital. It made the journey complete for them and I think has allowed for transition to begin without any problems. Cole, Terry, Lampard, Ferreira, Cech, know they did it and the club will have to move on one day. But they did do it.
So, depth is the key. So many quality players to replace if a player is out of form and all have talents that fit well