Vidic and Smalling
Rio has played 23 of the last 26 games to show that he is over his back problems. He could have done better against Tottenham, but the changes that Sir Alex Ferguson made against Newcastle showed that it wasn't just the centre-backs at fault.
As Gary Neville pointed out in a review of the Spurs match on Sky Sports, the full-backs had been too far from the centre-backs; and Evra and Rafael must not be upfield at the same time.
Together with a more mobile midfield and the re-introduction of Tom Cleverley, the defence was watertight at St James' Park. At last United had a clean sheet and the offense prospered.
Rio still reads the game extremely well and the only limitation he now has is that he has lost some of his pace. Sir Alex has suggested he use his ability to read the game to compensate, making smarter decisions.
Vidic's natural aggression and all-round physicality is probably why he is back in the physio room. He may not have been quite match fit, but as captain he was desperate to ease the centre-back injury crisis. He may have pushed his body too far and the knee could not take the strain.
He'll be back in late November at the latest, but Sir Alex now has two problems to resolve to cement the defensive solidity.
Playing a higher defensive line
One of the reasons why other teams have been able to put pressure on United's defence in early season is because it has been playing too deep.
There are two possible reasons for this:
First, when Paul Scholes plays, he sits too deep. He makes up for this with his ability to spray accurate passes, but Carrick is supposed to be the holding player. United's attacking style doesn't require two holding midfielders.
So a vacuum opens up between them and the front line, which Tottenham were able to exploit in the first half. And the attacking players either don't get the service they need or have to keep coming deep to get the ball.
A second reason is that when Vidic and Ferdinand play together, neither has the legs to be up on the halfway line one minute and back in the box cleaning out a pacy attacker the next.
Sir Alex wants his team to play a much higher line as they did so successfully against Cluj and then Newcastle. Apart from having a more mobile player like Cleverley or Anderson alongside Carrick, he needs to play a pacy centre-back.
Jonny Evans has been performing that role alongside both Vidic and Ferdinand this season. He loves to go forward like Mats Hummels does for Dortmund. His Achilles heel, however, is occasional lapses in concentration.
Against City in the 6-1 defeat last season, he was totally to blame for Balotelli's first goal that unlocked the floodgates. He had a "bad hair day" before finally getting sent off.
The pace to attack and defend
Once upon a time, not so long ago, Rio Ferdinand was that man. If anything, he should have attacked more from the back. He is a footballing centre-back who started life as a striker. He should score more goals.
The old Rio would have been able to bring the ball out from the back, allowing Carrick to move higher in the way he did so well for England against San Marino. That was also how he scored against Galatasaray.
But you can only be a marauding centre-back if you have the pace and legs to get back when needed. Especially when a Fernando Torres is around. The Spanish striker was always able to expose Vidic's lack of pace while he was at Liverpool.
Sadly, Rio doesn't have that pace or recovery ability any more. This doesn't mean he doesn't have a future at Old Trafford. He is the one player out of himself, Scholes and Giggs who should get another one-year contract. He is potentially the Paolo Maldini of United.
The value of having a footballer in the back line was shown on Friday, when Roy Hodgson was thinking about dropping Carrick in as a centre-back as the game progressed, to make England even more aggressive.
No need to look outside United
If Sir Alex was to look elsewhere to find the player who ticks all these boxes, he should look no further than Mats Hummels. Jan Vertonghen was a possibility before he went to Tottenham, but Hummels is the complete central defender. At Old Trafford he could become the best in the world.
Javi Martinez was in some ways the dream ticket before he went to Bayern Munich. He can play the footballing central defender but is also a world-class defensive midfielder.
But Martinez cost almost £34 million and Hummels' fee wouldn't be much less than that.
In any case, why look outside Manchester United when the solution may already be there?
The case for Chris Smalling
OK, there will be plenty of people at this stage saying "what about Phil Jones?"
The thing is that Phil is a very versatile player. Neither he nor Sir Alex yet knows what his best position is, but like Smalling, he can play at right-back. Of course he can play centre-back, but he has already shown that he may have many of the qualities Roy Keane had.
He is very strong; likes to come out of the back; can chip or hit a pass; score goals with foot or head. It is no surprise that he has been likened to the great Duncan Edwards, who could do everything. Phil is only 19 years old and we shall see how things develop.
But he and Chris Smalling have already been talked about as a future England pair. Whether that is at centre-back and right-back or as the central defence is yet to be decided when both get a run in the side.
When Chris Smalling first appeared for United there were immediate similarities with Rio Ferdinand. He looks the ideal replacement.
He can head a ball as well as Steve Bruce, has great pace, can play football and loves to bring the ball out from the back and attack.
He is still only 22. Two years before he joined United he was playing for Maidstone United and probably hoping one day to follow his predecessors and play for Gillingham.
He was 20 when he joined United after only 13 games for Fulham. He remained there on loan for the rest of the 2010/11 season. Nine months later, he was called up for England. Although he didn't make his debut until September 2011, he has looked a star of the future for both club and country.
He only played 16 games in his first season and 19 last season, when his playing time was curtailed slightly due to a groin injury. Since then, he has had nothing but bad luck.
Just about fit from his groin problem, he broke his metatarsal in training and was ruled out for a further 10 weeks. This was deeply frustrating for his manager because Vidic and Evans were also both out and Sir Alex had to draft Michael Carrick as an emergency central defender yet again.
Smalling is due to return this weekend against Stoke City. Rio will have had a two-week break due to not being called up, but Jonny Evans has been playing for Northern Ireland.
With Phil Jones and Vidic still a few weeks away, the likelihood is that Smalling will get a start alongside Rio at the weekend, giving Evans a rest (though he may well be on the bench). Alternatively, to combat the height Stoke have, the manager could field all three, with Smalling at right-back.
This is not a make-or-break season for Chris, because he has undeniable talent. If he gets a run in the side before Nemanja returns, either alongside Ferdinand or Evans, he can make a statement for the future.
He can expect an inevitable call-up from Roy Hodgson at some time, who already has the 2014 World Cup in mind (which is why he has "retired" Ferdinand). With John Terry also leaving international football, there is no shortage of contenders for both the centre-back berths.
Expect Smalling to be one of those in two years' time. In the meantime, if he shows the undeniable quality that Sir Alex bought, he can put down a strong marker to partner Nemanja Vidic until the United captain retires or moves on.
England will miss a footballing central defender now Rio is gone. Chris Smalling undoubtedly has a unique set of qualities to be that man. And with Micah Richards as the potential John Terry replacement, Chris can also be part of a strong and balanced partnership for England for years to come.