"I always say with United that if someone turns up who we can't afford to miss, you have got to do something about it no matter what position it is," said Sir Alex.
Sometimes a player emerges and you say `we shouldn't miss him'," he added.
"We were getting great reports on Vidic time and time again from our scouting process. We felt he is 24 years of age and a real defender why don't we take him? With Wes Brown we know that from time to time he gets injuries, and that is an unlucky part for him, but we have to protect the club.
"In defence now I am far more comfortable than I was last year."
"There is a bit of youth coming through with Phil Bardsley and Gerard Pique, plus we have the experience of Gary Neville and also Gabriel Heinze will be coming back after his injury."