Robin Van Persie & Jonny Evans
Manchester United captain Nemanja Vidic was an absolute giant against Reading in the FA Cup last night. This was truly a captain's performance, but he isn't getting any younger and succession is a real consideration.
One of the reasons is that Sir Alex does is not yet comfortable in playing his captain in every game.
Fortunately, in Patrice Evra, he has a vice-captain who hardly misses a game. Since the beginning of 2007/8, he has started 272 games out of a possible 331, more than any other player.
And for all those who sometimes question Evra's contribution, when he hasn't played this season, United have conceded 14 percent more goals and lost 30 percent more matches.
So between the two, Sir Alex notionally has a "dream team." "Pat" is undoubtedly a leader and his captaincy has been needed far more in the last couple of years due to Vidic's injuries.
Indeed, the possible doubts about the captain's full recovery are what now prompts a reasonable consideration about who would succeed him.
Both captain and vice captain are the wrong side of 30. Indeed, both are 32 this year and if there are questions about how much longer Ferdinand can play on, the same will soon apply to his colleagues.
Whether it was because of rebuilding after his injury; whether he has lost a yard of pace; or it is merely the sands of time, there were signs last night that Vidic is not quite what he was.
It was unusual to see him so many times feint and turn away from an attacker to buy time to make a pass, rather than bursting past him.
You don't become a bad defender overnight and both the captain and his best partner, Ferdinand, are playing with their heads more, literally and metaphorically.
There was an instant last night where Vidic was under pressure and, rather than risk losing the ball, simply ran the ball out of play. He does this as well as any central defender. He knows when to do the simple thing. This is something Smalling can learn from, for example.
But this was, nevertheless, a giant of a performance. Despite a shuddering clash of heads with Noel Hunt in the first half, Vidic headed clear time and again, fearlessly challenging where others would have shied away.
This is one of the two or three critical elements in the make-up of a great captain, and it is no surprise that they are often central defenders or central midfielders.
Before we consider the candidates to succeed Nemanja Vidic, let's look at some of the recent United captains and what they brought to the job.
Roy Keane is undoubtedly one of United's greatest captains ever. He brought every single critical quality to the job.
He was a leader, extremely vocal on the pitch, led by example, drove the team forward, was fierce in the tackle and an undoubted first choice who could do every job on the pitch except goalkeeper.
It is not Michael Carrick's fault that he is unfavourably compared to Keane. They are very different players and as Sir Alex has conceded, you can't tackle in the modern game in the way you could even five or six years ago.
No doubt, Keane's qualities are the main reason why some supporters are demanding Sir Alex signs Victor Wanyama from Celtic: combative, skillful, driving from back to front, scoring goals.
The last captain before Vidic was Gary Neville. He had the same driving qualities. You would never feature him in a list of United's most skillful players, and he scored hardly any goals. But he drove the team forward and kept the opposition at bay. He too could be fiery and vocal.
Indeed when both were in the team, together with Peter Schmeichel, you had a mighty noisy penalty area!
Eric Cantona was very different again. He was also much quieter, but he again led by example and, like Van Persie, was somehow able to produce a crucial goal when needed. Eric was very much an interregnum captain, between Steve Bruce's retirement and Keano's elevation.
Brucey also led from the front and there are many similarities between him and Vidic. Both utterly fearless; great headers of the ball and goal scorers; both highly vocal leaders.
Sir Alex's first captain was of course Bryan Robson, whom he inherited from Ron Atkinson. Again a very similar player to Keane and always ready to put his body on the line in the team's cause.
So there are many similarities between these captains, and all of them were defenders or central midfielders, with the exception of Cantona, who was clearly the best player in the team at the time.
Before Sir Alex, things were very different. While George Graham, Martin Buchan, Noel Cantwell and Bill Foulkes bore similarities to Vidic, Denis Law and Sir Bobby Charlton were up front and Willie Morgan was a winger.
We are unlikely to see a winger captain United any time soon, which is a pity. Ten years younger and surely Ryan Giggs would be a "shoo-in" to be next captain.
Rio Ferdinand has all the qualities to have been a top Manchester United captain and surely he would have been a better choice than John Terry for England?
He is a good person who does great charitable work such as in Africa. He is highly respected within the squad and in the game itself.
He is technically one of the best footballers in the squad, having started life as a striker. Indeed, many likened him to Franz Beckenbauer in his younger days. He would have fit in perfectly as a sweeper at Bayern Munich.
He also is very vocal, fearless and usually utterly reliable. Even now, in their golden years, fully fit, Vidic and Ferdinand are surely United's best centre-back combination. We may see that in the home leg against Real Madrid.
So here are some key qualities of the ideal captain:
Unquestionable first-team selection every match;
Vocal; a talker/leader on the pitch;
An utter professional on and off the pitch;
Intelligent, thoughtful and eloquent as appropriate.
So while this is a somewhat idealistic list and should not exclude strong candidates who don't possess all these qualities, let's look at the possibles at United.
Wayne Rooney has most of the qualities needed to be a great captain at Manchester United. Surely he would jump at the chance. If he was moved into a central midfield role, he would surely be a strong candidate.
He is probably the nearest thing United now have to a Roy Keane. He has cleaned his act up in terms of his previous mercurial behaviour, especially with England.
Ironically, he seems more likely to become England captain than Manchester United.
Why? Two main reasons: his demand to leave a couple of seasons ago must surely have tested Sir Alex's patience and rattled the bond between them; and also the small matter of Rooney's past dalliances off the pitch.
Whether for United or England, the press would have a field day dragging up all the dirt from the past and asking those very questions if Rooney were appointed captain. That doesn't mean he isn't a very strong candidate for vice-captain.
We have looked through the squad for other candidates, assuming that the next captain will come from the present group and there are relatively few real candidates.
Of course a key consideration is when Sir Alex decides to "retire" Vidic. That might come even if he didn't leave the club, if he cannot play regularly as before.
And Evra would surely not be promoted because the sensible thing would be to appoint for at least the next two or three years.
The other candidates appear to be, in no particular order of importance: Jonny Evans; Phil Jones; Tom Cleverley; and Robin Van Persie.
Surely Sir Alex sees Jonny Evans as a future captain. He is already vice-captain of his country. He is one of the best central defenders in the Premier League and on current form, arguably United's number one.
Phil Jones has been touted as a future captain for club and country almost from the moment he landed at Old Trafford. He also has the fearlessness that characterises other great captains and will surely be a certain starter in a year or two.
He has been compared to Duncan Edwards, who captained England Schools and many people think would have been captain at the 1966 World Cup had he lived. Phil Jones is undoubtedly a future captain but he is still only 21 in a couple of days time.
His time will come, but there are better candidates for the time being.
Tom Cleverley also has leadership qualities but he, too, is maturing as a player. He also will surely be a first-choice player in years to come and can only get better.
But the one player who is a stand-out to be the next United captain, despite only just arriving at the club, must surely be Robin Van Persie.
Robin Van Persie
When history is written, Van Persie may yet be seen to have been Sir Alex's greatest signing ever.
If United win the Treble this year, he will be the reason.
When you compare him to the list of qualities proposed in an earlier slide, he ticks all the boxes.
It is no accident that he was Arsenal captain. He didn't get the job on "Bugginses turn." It wasn't for blind loyalty. He was unequivocally the best player in the Arsenal team.
Who would disagree that he is now the best player at United?
He gave Arsenal eight seasons, during which he won just two trophies: the FA Cup and the Community Shield. And both of those in his first season. That is extraordinary loyalty and commitment.
He came to United because the "little boy" inside him to told him to; and because United "breathe football." He loves the "buzz" of playing for United and wants to be in every game.
He is as fit and strong as he has ever been and surely he is playing his best football. He will surely already have learned much from Ryan Giggs and Rio Ferdinand about how to protect his body and extend his career.
He may only be on a four-year contract that will take him until just short of his 34th birthday, but surely United would not hesitate to offer him a rolling one-year contract after that if he remains fit?
Robin Van Persie is a winner. He is also a quietly spoken but nevertheless captivating man. When you watch him play, he is like Wayne Rooney, a young boy playing in a man's frame.
And there the comparison ends. While both are intelligent in a footballing and a general sense, there is still a rawness about Wayne. He might get away with "f***ing and blinding" as a player, but it won't wash as captain.
Van Persie is the "professional's professional," like Vidic, Ferdinand and Giggs. He looks just as much at home sitting beside Sir Alex in a press conference as he does slotting a goal for United.
Indeed, there are so many qualities that he has and the case is so strong for him to be the next Manchester United captain that we shall leave you to review the previous list and make your own mind up.
One thing is for certain. He will be a first choice as player and striker for as long as he stays at United; he will never let the side down and will continue to lead from the front. It's the only way he knows.
And he's pretty combative in the tackle, too!
The only question that remains is who should be his vice-captain (and probably successor). Partly because he is now arguably first choice in defence and also for his many other appropriate qualities, that man should be Jonny Evans.