Well, loosing any one of Vidal, Pirlo or Tevez would equally make us much worse, but that's nothing to worry about because it's normal. Show me a team that would not struggle while without their key players?
It really is the same with every top team there is. Ronaldo, Messi, Aguero, Ribery...The only team not reliant on a single player IMO is Dortmund.
I dare say we even fare pretty well in that regard, as we have Vidal, Pogba and Pirlo to step up.