In the end, the best that a Living National Treasure can hope for is a little mercy. When a Living National Treasure has some kind of public disaster, he certainly doesn’t get instant forgiveness. Just a little tenderness, a little regret — if he’s lucky.

But mostly we just wag our heads at the ironies of it all, express bewilderment at the fact that a person can excel in one area of life and then fail in another. Amazing: the attainment of sporting excellence doesn’t make you a perfect human being.

Martin Johnson, the England rugby union team manager, must be wondering how much longer he can rely on his record as an LNT. He has been in the job for more than a year. Now, after a desperate display against Australia on Saturday, one in which most England attacks went sideways and/or backwards and in which most kicking was more about the offloading of responsibility than the building of a coherent play, Johnson has two more autumn internationals to save himself from public dismay, and perhaps worse.

As he deals with injuries and his lack of experience, we have all seen the fading of our memory of the never-a-backward-step leader of the team that won the World Cup. Instead, we look upon a bewildered bear dressed incongruously in a white shirt with cuff links. Who’d have thought the LNT would be a cuff-link man?

The debunking of LNTs is not something that we Brits go in for as a matter of course. We leave Tall Poppy Syndrome to the Australians. If we must demolish a hero, we generally do so with regret.

But there comes a point when the protection afforded to LNTs runs out and Johnson is perilously close to that point. Either England will put on a couple of decent performances against Argentina and New Zealand in the next eight days or the dam that holds back the flood of criticism — the dam that only an LNT can construct — will start to crack.

Then what will happen? Johnson is an expert on sporting history, so he will know all about Ted Dexter and Kevin Keegan. Both were LNTs; both became, in different ways, ridiculous. When the dam broke, they were swept away.

Dexter was a wonderful batsman. He scored nine centuries for England, including six very big hundreds. But perhaps his best innings came when he went out to bat with England nought for one against Wes Hall and Charlie Griffith in 1963. Dexter took that fearsome West Indies attack apart, scored 70 off 75 balls and was loved everywhere for it.

He became chairman of selectors. He spectacularly but calamitously dropped David Gower and Jack Russell for the “Brownwash” tour to India in 1992-93 and the next summer England were again thrashed in the Ashes. Dexter, an intelligent man, committed the unforgivable error of making a joke. He put England’s failure down to misfortune: “We may be in the wrong sign ... Venus may be in the wrong juxtaposition with somewhere else.”

No one is more humourless than a disappointed sports fan. The jest proved that Dexter was not only bad but mad. By now, no one remembered his status as an LNT, his patrician swagger and his glorious, aggressive batting. He was simply a liability and a laughing stock. When his resignation was announced on radio, the crowds at Edgbaston heard it and cheered.

Kevin Keegan was the England football captain. He scored 21 goals for his country during a dark and difficult time. But it was more for his great club record that he acquired his LNT status. He played for Liverpool and made that rare transition to become a successful player on the Continent, playing for Hamburg and twice becoming European Footballer of the Year.

He was seen universally as intelligent, adaptable, strong, a person capable of making the best of himself and the circumstances he found himself in. A decent and likeable man, he represented all the best things about English football.

But for him, too, the dam broke. He became England head coach and led them through a woeful performance in the 2000 European Championship finals, losing to Romania and Portugal and failing to reach the knockout stages.

Worse followed as the qualifying tournament for the 2002 World Cup began. England celebrated their last game at the old Wembley with defeat by Germany. They were booed off the pitch. Keegan wept and resigned, admitting that he wasn’t good enough.

This was not without courage and honesty, but he was given little credit. Instead he became ridiculous. Football, of all sports the most brutal and superficial in judgment, had found him wanting. It was as if his heroic days had never been.

One can only respect the great players who go into management rather than opting for punditry and the celeb circuit. They all willingly and knowingly risk their LNT status.

Sir Bobby Charlton and Bobby Moore had brief and disappointing careers in management. But it is when you get involved with the national side that you find the real dangers. Here, the emotions that attach to failure are stronger and more vindictive. Even hugely successful managers suffer when things go wrong: Sir Alf Ramsey, knighted for his achievements after guiding England to the World Cup in 1966, ended up sacked and ridiculed.

But I am more concerned here with great players and the shift in perspective that comes when they forfeit respect. It’s not only management that can have this effect: Lester Piggott went to prison for tax evasion; Andre Agassi has created a sensation by admitting that he took crystal meth; O. J. Simpson found a still more dramatic way to lose the adoration of his public.

Not all examples of wrongdoing cost LNT status. Tony Adams went to prison for drink-driving after an episode that could have killed people; he was cheered to the echo on his return. John Terry parked his Bentley in a disabled parking space, a perfect symbol of the insolence of the modern footballer, and he wears the armband of the England captain.

If you choose to enter the badlands of coaching and management, then clearly and splendidly you are prepared to put your reputation at risk. To retain the restless quest for achievement when you have already conquered the world, that is something to cheer.

Johnson stood on the summit six years ago, monarch of all he surveyed. Australia were vanquished; England, Johnson’s happy few, had conquered the world. Johnson was the master of the universe — and he still wanted more. Let us cheer him for that: a man who won everything and was still unsated.

But now he has reached a time of immense difficulty. Even in the comparatively generous world of rugby union, honeymoons come to an end and the generosity given to an LNT wears out.

Can Johnson turn it all around? I hope so. It would be a wonderful tale to tell.

But right now he reminds me of the ruined statue that stood in the desert. The one that bore the inscription: “Look on my works, ye mighty, and despair.”