Here's hoping you're completely wrong but I fear the worst. Nemanja isn't the type of player to go down and stay down unless something is seriously wrong and he did look in agony when being taken off. The replay in slow motion doesn't fill me with much hope either. He'll be a massive loss in more ways than one how ever long he's out for.
Vidic injury blow for United | Manchester United News, Fixtures, Results, Transfers | Sky Sports
"United boss Sir Alex Ferguson revealed after the game that he fears Vidic has suffered medial ligament damage, which could sideline him for a significant amount of time."