Drogba no doubt. They're both monster players, but the Drog is unplayable at the moment. They can get by with the strength in depth they have in midfield.
Chelsea's midfield is so solid they can operate without Essien. Yes, he will be missed, but with Ballack, Mikel, Lamps, Deco, Malouda, Zhirkov and even Kalou (on the right), they'll get by. Anelka is still on fire and Chelsea will play with five in midfield in Drogba's absence. You never know Sturridge might do a Walcott and creep into Capello's WC squad (I don't actually believe this).
Don't jump on the bandwagon, Chelsea are the team to beat, with or without their African stars. United the only team who can catch them... gets boring doesn't it?