Although, in another article, I placed the blame firmly on Jonny Evans and Patrice Evra, many have suggested that Rio was to blame and that he's 'past his sell-by date' at age 32.
Personally, I don't subscribe to that theory. Rio has always had the vision and speed of thought to be one step ahead. It is the main reason why he has been such a great centre back; and is also why he can look like he's taking the game easy.
Sir Alex has quite rightly recommended that Rio take account of his loss of pace and adjust his play accordingly. That doesn't mean Rio should be pensioned off, however.
Paolo Maldini played for Milan until he was 41 and Fabio Cannavaro has only recently retired, having played for Juventus and Italy until he was 37.
Yes, the Italian Serie A can seem more sedate than the Premier League at times, but it can also be just as hard, as Gennaro Gattuso and Marco Matterazzi have shown.
Among those clubs said to be interested in signing Rio are Roma and Tottenham Hotspur. Harry Redknapp isn't a bad judge of beefcake and, if he wants Rio, why would Sir Alex want to let him go....?
However, rumours suggest that United are unwilling to discuss a contract extension to his 2013 deal.
Part of Sir Alex's long term strategy has always been to have young talent coming through. Here we discuss possible younger Rio 'lookalikes' who the Boss might sign in January, together with the potential replacements already on the books at Old Trafford.
Hmm why not? This rumour keeps coming up—and it has some legs. Why? Because Subotic and Vidic have played together for Serbia.
Although Nemanja has now retired from international football, he is well-placed to advise Sir Alex on his fellow national. With rumours also flying about Vidic being a possible target for Real Madrid, you can look at it either way.
Subotic could come in and play alongside Vidic; or he could replace him. More likely he would be the short or longer term successor to Rio—whether or not Ferdinand stays.
Neven Subotic is the right age (22), with the right experience. He is an established Serbian international with already 28 caps at a young age for central defenders.
He also has 129 appearances for Borussia Dortmund, with 12 goals—a stat that will appeal to Sir Alex, because Rio hardly ever scores and took three years to net his first.
Apart from the obvious reasons why he would come to Old Trafford, including the Serbian connection, why would he leave Dortmund?
A year ago, Chelsea were strongly rumoured to have had a £17.5 million bid turned down by Dortmund. The Blues turned to David Luiz instead, on the last day of the window.
While the German side went on to win the Bundesliga in 2010/11, they are struggling in the Champions League and seem unlikely to qualify for the knockout stages.
That may make it more likely that United could bid for Subotic and/or Mario Gotze, who would thereby not be cup-tied if the Red Devils progress.
Sir Alex Ferguson has cast envious eyes over Jack Rodwell since he was 16. Originally and primarily a midfielder, there are many, including Rodwell himself, who believe that he will end up as a centre back.
This makes him doubly attractive to United because, as with Phil Jones, Rodwell could play either at centre back or defensive midfield, where they are currently desperately in need. Like Rio, he is a ball-playing defender with good vision and awareness.
Jack has just been called up for England, which says a lot for his character, because his form was indifferent in 2010/11 to the extent that Sir Alex apparently let his interest wane, having previously been rumoured to be prepared to pay up to £20 million.
Everton have financial challenges and, as well as being unable to sign new players, may need to raise significant funds to keep the bank manager happy.
Rodwell could achieve both. His manager, David Moyes, is not only a close friend of Sir Alex, but also one of those tipped to ultimately succeed him. He would give the lad a good write up, if his recent comments are a fair reflection.
Ironically, Jack has been called up as a midfielder but, as with United, he is potentially doubly valuable to Capello in the European Championships.
You may not have heard of him but, if Fergie is considering a ball-playing centre back in the mould of Rio to succeed him, he may like this...shades of David Beckham. Xabi Alonso described it as the ‘greatest goal ever scored at the Anoeta Stadium'.
But while Martinez had previously scored with a cracking volley for the Spanish U21s, he is more renowned at present for his cheeky backheel for the Spanish that embarrassed his potential teammate at Old Trafford, David De Gea, by nestling for an own goal.
Martinez is 20 and has already scored three goals in 11 appearances this season for Real Sociedad, for whom he is an established first choice.
His form so far this season is impressive. He has a better tackling and interception rate than his peers at Barcelona, which may explain Pep Guardiola's apparent interest. He is, like Ferdinand, a ball-playing centre back, averaging 50 passes per game.
Rated by some as the best young centre back in Spain—who must be the world's best team right now—it would be a feather in Fergie's cap to nick him from under the noses of his European rivals, having lost Gerard Pique back to them a couple of years ago.
Manchester United are, above all, a footballing side. That's one of the reasons why Rio has done so well, although some would wish he had played from the back more often.
Mehdi Benatia looks like he fits the bill as a ball-playing, goal-scoring, tough tackling centre back, as the above video shows.
While he is a little longer in the tooth than some of the lions we've showcased on here, he has had to reinvent himself since he almost disappeared at Marseille.
Now with unfashionable Udinese, he was recommended to Arsene Wenger last season by Samir Nasri, who played with him at Marseille. He has done much better with the Serie A side but would clearly love to play in the Champions League.
At 1.88 metres, he is a similar height to Rio but, as the video shows, is also comfortable playing from the back and scoring goals. This season at Udinese he has two goals in 14 appearances.
More important, he has dual nationality—Moroccan and French, meaning there would be no difficulties with a work permit.
Previously linked with Arsenal, Barcelona and Liverpool, there is a rumour doing the rounds that the former are still interested in Jan Vertonghen of Ajax. This would be unusual now that Vermaelen has returned, especially with both Koscielny and Mertesacker showing better form.
Also 24, Vertonghen is primarily left-footed and can also operate as a defensive midfielder. Presumably, like Jonny Evans, he could cover left back for Manchester United, depending on how quickly Fabio becomes established.
He is a Belgian international with 23 U21 caps and already 34 appearances for the senior team and two goals.
At Ajax for five seasons, including a loan spell away, he has 182 appearances and 16 goals. He was Thomas Vermaelen's partner before the latter moved to Arsenal.
Valencia tried to sign him on deadline day and Inter are interested, but in September he was quoted as not wanting a move before next summer.
If Sir Alex does have an interest, the critical factor could be whether Ajax make the knockout stages of the Champions League. Jan is unlikely to leave before summer if they do, and he would be no use to another team due to disqualification.
So, the questions are: would Barca let another of their rising centre backs go after they lost Gerard Pique to Manchester United? And how does Bartra feel, not only playing second fiddle to Pique and Puyol, but not even being able to get in the team when one of them is out, while midfielder Mascherano fills the gap?
Unless, of course, Pep Guardiola is lined up to succeed Sir Alex Ferguson...
Tottenham are currently, and Chelsea previously, rumoured to have an interest, e.g.
Bartra is also a ball-playing, right footed centre back. Aged 20, he stands 1.84 metres but he has, as yet, only featured three times for Barca, despite 24 appearances for Spain at various levels up to U21.
Almost a year ago, it was claimed that he had been offered to Real Madrid; a ruse that may have been designed to secure him the new contract that he now has, but only until next summer.
Such things can turn a young player's head, however, like the current rumoured situation between Paul Pogba and United. It was said that Spurs would make an offer for Bartra before the August deadline, which came to nothing, but if the story is true, he is prepared to consider a move away from Barca.
With the style of La Masia flowing through his veins, surely, Fergie would welcome him with open arms.
The rumours that Barca are interested in Mats Hummels from Borussia Dortmund probably won't have pleased Bartra either.
It seems that whenever Neven Subotic's name is suggested as a transfer target, Mats is not far behind. They are both ball-playing centre backs, who have formed a formidable partnership for Dortmund.
While the German 2010/11 Bundesliga champions are unlikely to let both players go, an early exit from the Champions League could force their hand for one of the two.
Again, a possible attraction for Manchester United is that Mats can also play defensive midfield—a great attribute for ambitious players to have on their cv!
Hummels is the right age at 22, but if he's prepared to turn down Bayern Munich, why would he join United? Mind you, he came from Bayern, but the release clause in his new contract is believed to be at least £16 million. Juventus are also said to be interested.
At 1.92 metres, he was a product of Bayern's academy, making only one appearance in their first team before going on loan to Dortmund.
Since becoming a permanent member of the latter squad, he has scored an impressive ten goals in 72 appearances and has played ten times for his country.
I make no apologies for continuing to suggest that Gary Cahill would suit Manchester United. Every picture tells a story and some might think that his fearless defending makes him a more obvious replacement for Nemanja Vidic.
Gary can play football and score goals. It can't be much fun to have so much reported interest in the summer that didn't materialise; have to knuckle down for another season; and find yourself in a relegation battle.
Now they have thumped Stoke City, and the interest seems to be evaporating. One by one, the potential predators seem to be drifting away:
Maybe it's a bluff, because Gary is out of contract next summer. But, while United's name has been hardly mentioned, and he is older than any of our other targets, their proximity to Bolton, as with Phil Jones and Blackburn, could be a trump card.
With Jones showing he is the new John O'Shea; Smalling as comfortable at right back as in the centre; and Jonny Evans also a potential reserve to Evra at left back, there is plenty of room for another centre back. Vidic and Ferdinand aren't getting any younger.
If Gerard Pique can return to Barcelona, Ryan Shawcross could return to Old Trafford. However, it's not likely.
Some might wonder why Ryan was let go, having been with Manchester United since he was 15. But maybe it was more a case of feeling wanted by Stoke, having had a successful loan spell with them. With an established central defensive partnership ahead of him, plus Wes Brown and John O'Shea may have also swayed him.
Sir Alex is no mug, though. Having sold the lad for an initial £1 million, the fee has already risen to £2 million, but the Boss also has first option on Ryan if anyone else makes an offer.
Whether that will ever happen is another matter. The fee for an established Premier League centre back with a reputation for being tough and unyielding in defence is unlikely to be less than £10 million. Also, at 24, Ryan has yet to make a full England appearance.
There are better prospects elsewhere in this article.
Why look elsewhere when you have these two, still developing? Chris Smalling is a goal-scoring centre back and Phil Jones won't be long in joining him.
When Smalling first joined Manchester United, he looked tailor-made to follow in Rio Ferdinand's footsteps. Standing at 1.92 metres in his socks, he is a commanding header of the ball. He also looks like a football-playing defender—especially shining since he has stood in as right back in Rafael's absence.
If Chris is the new Rio, some have said that Phil is the new John Terry. He certainly has a similar style in the way he runs and attacks the ball, but there the similarities end. Jones is already a far better footballer. OK, Terry scores goals, but you wouldn't expect him to go on the sort of 60-yard run and shot that Phil did last week in the Champions League.
With Jonny Evans also highly rated by Sir Alex, some would think there is no need for another signing to replace Rio. But the manager likes and needs flexibility. He has a potential defensive midfielder in Jones; a replacement left back in Evans; and a right back in Smalling.
Sooner or later, Sir Alex will have to decide on a settled formation for after Vidic and Ferdinand. If he is going to build around the above three, he also has plenty of potential in United's youth.
So you could argue that, injuries permitting, he has no immediate need in January and no need at all next summer, as loanees return and other prospects grow.
Ezekiel Fryers (Zeki) probably can't believe his luck. With arguably the best up and coming United centre back out on loan to Peterborough, Zeki has had the chance to show his rich promise already at just 19. He does, however, have 13 appearances for England at various youth levels. He can play left or centre back.
He is far from the only promising centre back coming through at Carrington. As with other positions, the next couple of years may see the emergence of several stars of the future. If so, they could save United a small fortune in the transfer market.
Nevertheless, Sir Alex loves to blend youth with experience and transfer buys with home grown produce. While Ryan Shawcross slipped away, there are at least a couple of other centre backs who could be at least as good.
Scott Wootton is very highly rated at Old Trafford, which sort of makes a nonsense of any transfer rumours in central defence. He was actually a product of the Tranmere and Liverpool youth systems, who became disillusioned at how many overseas players Rafa Benitez was importing.
He swiftly became captain of United's Academy side and could well have captained the Reserves if Oliver Gill hadn't been around—which he's not now. Scott has now returned to Old Trafford with a knee injury and who knows...he might feature in the New Year.
Among the others who should make it at the highest level, I rate Reese Brown as even better than his brother Wes Brown at the same age. He is also a footballing defender.
Sean McGinty was one of the most highly rated young players ever at Charlton Athletic. At 15, he was Ireland's under 17 centre back and has already gone onto play several times for the Irish U19s. Signed by United at 17 for a reputed £500,000, he is also the most highly rated of the current Irish crop.
Tom Thorpe captained the United team that won last year's Youth Cup, beating Liverpool and Chelsea along the way. I can't improve on this profile of Tom who, in my opinion, is possibly the best of the lot, especially being not only a footballing defender, but also versatile enough to play all across the back and even in midfield.
Blessed by a truck load of useful defenders, it is hard to predict who will actually make it in the first team at United from the current crop of young men. The final two to mention are Michele Fornasier and Michael Keane.
It may be that Michael will concentrate on being a left back, but he has the height and composure to play at centre back and has already turned down an Irish call-up. He played well in his debut against Aldershot in the Carling Cup.
Michele Fornasier was already Italian U16 captain when United nicked him from Fiorentina. He partnered Tom Thorpe at the centre of defence in the successful Youth Cup campaign and has been a regular starter since he joined.
So, all in all, United seem to have most of the bases covered going forward, especially at centre back. It is no surprise that Sir Alex is trying to build a squad of versatility, because in the past, title runs have been almost derailed by injury crises.
As this season has already shown, players will get their chance on a regular basis if they show their flexibility. If you look at the model set by Barcelona in Europe, the modern footballer has to be able to do almost everything, interchanging across and up and down the pitch.
Rio Ferdinand was ahead of his time, like Bobby Moore, in being a footballing centre back. In our opinion, that is the way forward, to a form of 'total football' that can match Barcelona and Real Madrid. That is the challenge for the young players who are established and those trying to break through.
The future is bright...the future is Red...