Ferdinand has starred the Premier League champions since joining from Leeds United in 2002, but has struggled with injuries in recent seasons.
However, when fit, the former England captain was one half of a formidable pairing with Nemanja Vidic at the heart of United's defence and helped the club to win a record-breaking 19th title last season.
His role as a first choice, though, is now under threat after Chris Smalling moved to Old Trafford from Fulham in 2010 and was followed through the door by fellow England rookie Phil Jones this summer.
Ferguson's decision to drop Ferdinand for Saturday's win at Everton, on the back of the 6-1 thrashing by Manchester City, has fuelled speculation about his future at the club.
The former West Ham man, who turns 33 next week, will replace the suspended Vidic for Wednesday's Champions League tie at home to Otelul Galati.
But, while 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."