Ferguson is keen to retain Ferdinand for the long term, he suggested that the stopper is not an automatic choice in his team.
When asked if Ferdinand and Vidic are his first-choice pairing when fit, Ferguson said on Sky Sports News: "What I explained to Rio the other day is simply this: That we have two young centre-halves coming through and I'm have very happy to have four of them, and I hope to keep the four of them for a long time.
"Rio is almost 33 now and obviously he has lost a yard of pace he had five years ago. That doesn't mean to say he can't tailor his game in a different way.
"Whereas he used to rely on his pace, he doesn't do that now, he can't do that now, that's obvious.
"We've all faced that deciding moment in your career when you realise you have to change your game. I did it myself as a player when I lost that real sharpness and tailored my game differently.
"Rio will do the same and he'll be fine. He will still play a big part for us."