On the Premiership:It is not just in the House of Windsor that succession has become a thorny issue. Football's aristocracy are also being made to confront some unpleasant truths, and you could sense the fear in Bobby Charlton's voice last week when he mused on who might follow in the Yeti-sized footsteps of Manchester United's Alex Ferguson. "You can't plan for something like Alex retiring," was Charlton's non-conclusion.
As a United board member, Charlton will presumably not be able to keep his head buried in the Manchester silt forever, but his reticence is understandable. The 69-year-old is old enough to remember the club's cack-handed efforts in replacing Matt Busby when he moved upstairs to a general manager's role in 1969.
His replacement, the reserve team coach Wilf McGuinness, struggled on for five painful months, his every move haunted by the spectre of Busby in the Old Trafford directors' box. His predecessor eventually swept back to centre-stage while McGuinness wound up at lower league York City, a bitter and broken manager.
Ferguson would not win many popularity contests outside Manchester, and London's Metropolitan police can hardly have been short of suspects while investigating who punched the Scot in the groin at Euston station last week. But at United, his home for the last 21 years, he is rightly revered.
The club have already been spared the decision over Ferguson's successor once, in 2001, when the Scot decided to postpone retirement, but they will not be so lucky a second time. Ferguson turns 66 on New Year's Eve and even this personification of Celtic cantankerousness cannot make time stand still.
Whether they like it or not, United will soon be scouring football's classifieds for Fergie Mark II.
It is an onerous responsibility, but at least United are not suffering alone. Everywhere you look in the Premier League, clubs are tormented by the realisation that they must one day replace the irreplaceable.
Arsenal may still be celebrating Arsene Wenger's decision to sign a new three-year contract, but there will come a time when the Frenchman keeps his pen in his breast pocket. Rafael Benitez's angry outburst at his board following last season's Champions League final suggested he will not think twice before walking away from Liverpool, and, for all their bitter squabbles, Roman Abramovich will quickly discover how hard it is to find an adequate substitute for Jose Mourinho when the mercurial Portuguese stalks out of Stamford Bridge for the final time.
It is not just results and silverware that the Premier League's elite four must replace. The best managers mould clubs so completely in their image that they appear less like autonomous institutions and more like the extensions of their leaders' personalities. Witness, for example, the swift, supple playing styles of Manchester United and Arsenal, the polished pragmatism that has become Liverpool's trademark or Chelsea's muscular defiance. Each side has become a living, breathing monument to the peculiarities of its creator, and while such edifices take years to build, they can be destroyed in the time it takes to hand in a resignation letter.
Managers can acquire a rarefied air in less exalted places. Charlton Athletic supporters never believed they would pine for the staid, stable days of Alan Curbishley after 10 seasons in charge, but last term's catalogue of disasters, culminating in relegation, was a reminder of his worth. Elsewhere, Bolton are discovering the pitfalls of life without Sam Allardyce, a big man who took a small club to hitherto unscaled heights.
They are both respected figures who loom large over the Premier League, but the nation's football landscape is defined at the top flight's summit rather than in its foothills. The failure of any of the fabled Big Four to adequately replace Ferguson, Wenger, Benitez or Mourinho when they eventually depart would result in radical upheaval: a league which has been rightly pilloried for its wearying predictability could suddenly be thrown wide open.
However, a word of caution. A more open Premier League might have novelty value, but a significant factor in its current appeal lies in seeing four genuine managerial giants - with their wildly divergent personalities and working practices - attempting to out-do each other. It is one of the reasons Sky Sports will bill this Sunday's seismic meeting between Manchester United and Chelsea as a showdown between Ferguson and Mourinho rather than, say, Cristiano Ronaldo and Andriy Shevchenko.
The television schedulers should make the most of these iconic figures while they can. One day, possibly quite soon, they will bid farewell to the league they have turned into their playground. And English football will be the worse for it.
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