From what I understand Vidic will be only be missing for up to 2 weeks, and reports are saying that the injury isn't as bad as first thought, Ferdinand I heard 6 weeks but to be honest this season I don't think he will be as much as a miss, don't get me wrong he's still a top class defender but at 32 with regular injury problems I suspected he would be rotated a lot this season anyway, and Smalling is more than capable of playing alongside the experience of Vidic when he's back, and Evra should be back for the Spurs game which is a huge bonus.
As for De Gea, I don't think anybody was naive enough to think he was going to come into this league at 20 and be as good as Van Der Sar, but with a good set of coaches behind him every single game (and every single mistake) will give him crucial experience that every keeper needs, I still have no doubt that he will be a top class keeper in a few years time and will, as Casillas said, be pushing for Spain's number 1 shirt.