Club captain Vidic is slowing up, growing old and no longer invincible.
Thankfully, his reading of the game is better than ever, meaning that he can usually cover for his lack of pace by anticipating defensive crises and incoming threats before they emerge.
His aerial abilities remain fearsome, as does his determination to marshal his defence into position and keep concentration levels high.
Unfortunately, this managed decline will not last forever, and a sudden downturn in reflexes, sharpness or fitness could be just around the corner.
Evans, Smalling and Jones are by no means lacking physically and can be combative guardians of David de Gea's penalty box when they need to be, but none of United's young centre-backs can hold a torch to their superior, who remains arguably the best stopper in the world when fit.
In the case of Smalling and Jones too, United's central defenders are often required to fill in at right-back and in midfield, leading the squad's apparent strength in depth at the back to be far slimmer than first appearances may suggest.
That may mean that a new face will be required to replace the brawn and brutality of Vidic in the first team.
Surprisingly, there do appear to be options available in the transfer market, although prices will be reassuringly high.
Kurt Zouma is a long-term target for a number of Premier League clubs, including Manchester City and Chelsea.
And with St. Etienne's 19-year-old currently serving a 10-match ban for a horror tackle which left Sochaux’s Thomas Guerbert with a broken leg, as reported by Bob Treasure of the Daily Mail, the French youngster may appreciate an escape route in January, although he wouldn't come cheap given the competition for his signature.
Borussia Dortmund's Neven Subotic is another player who has been linked to the Premier League elite, and United especially over the years, although with the Serbian currently out through injury—and Jurgen Klopp determined to stem the flow of players from his squad—a move in 2014 may be unlikely.
Meanwhile, Sergio Ramos has reportedly been made available for €60 million after falling out with the hierarchy at Real Madrid, according to Marca. The Spaniard can be something of a loose cannon due to the amount of cards he picks up, although he may find that the referees of the Premier League are more tolerant of his style of defending.
A move looks unlikely unless United could come to some sort of arrangement to drastically cut Real's transfer valuation.
Sheffield United's Harry Maguire is the budget option, with the Mirror reporting that the 20-year-old could be available for just £4 million, although he would likely be more of a back-up than a main starter for the club.
The men who could add steel to United's defence
- Kurt Zouma—impressive
- Neven Subotic—injured
- Sergio Ramos—exorbitant