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.