Overview: Whether Keane's sudden departure was the result of an aggregate of small rows or one big one we may never know, writes Tom Humphries

It comes back to the story of the frog and the scorpion. The scorpion asks the frog for some help crossing a streaming river. The frog takes a look at the scorpion's poised stinger and politely demurs.

"Don't be silly," the scorpion says, "sure, if something happens to you, I'll drown too."

Persuaded by the logic, the frog puts the scorpion on his back and hops in. Halfway across, the scorpion stings the frog.

The dying frog croaks, "Why would you do that? You know that you'll drown now."

"It's just my nature," gasps the scorpion as he disappears beneath the water.

So. After yesterday's mutually assisted suicide by drowning at Old Trafford, the inquest must decide who was the frog and who was the scorpion? One thing seems certain: Keane's departure signs the professional death warrant for his friend and mentor. It was in the nature of both men that their relationship would thrive on success but die in a time of failure.

Our understanding of Keane is imperfect and blurred by the partisanship and divisiveness which the mere mention of his name provokes. As such, we've never really understood what went on in Saipan and we'll probably never fully understand what has gone on at Old Trafford recently.

There can be no doubt that the ruptured friendship between Keane and Ferguson is at the nub of yesterday's events, but as to whether Keane's sudden departure without letting the door slap him on the backside was the result of an aggregate of small rows or one big one we may never know. The statements from player and club yesterday had the air of mutually agreed settlement.

It seems unlikely, even by the zany standards of moral consistency which exist in modern professional soccer, that Keane's departure has anything to do with his disastrous expedition into the world of punditry on the state-controlled MUTV recently.

Much of what Keane actually said back then has been taken apart and twisted by an anxious media, but even in their distorted, hype-fed form those words ring true and went to the heart of what is ailing Manchester United. If United were looking for an excuse to get rid of Keane, the fallout from their cowardly jettisoning of an interview they had commissioned and conducted was hardly the time or place.

Historians and Keanologists are likely to attach more importance to the MUTV episode as a factor in altering Keane's mindset about his employers. The muffling of a fairly honest appraisal of United's shortcomings (and the leaking of a version of Keane's comments by somebody in the club's employ) would be seen by the player as a symptom of the strange values and remarkable self-delusion of the club.

If Keane, a proud professional, really did turn up for a reserve game against West Bromwich Albion on Wednesday night only to be told that he wasn't required and should speak to the manager, he was effectively being constructively dismissed by an employer who no longer had his respect. Twelve years of unsurpassed glory and influence thus came to an end.

What an extraordinary journey it has been though. Roy Keane and Alex Ferguson established a deeply symbiotic relationship many years ago and they became the axis around which everything at Old Trafford revolved. Two working-class men from Mayfield and Govan found they had sufficient common drive and passion to replicate each other on and off the pitch. They became the people around whom the world's greatest football club revolved.

For the guts of a decade newspapers got good fodder from predicting that there would be an industrial-strength falling out between the pair. The end of the world was always nigh. Eventually it came to pass.

In the meantime, though, Keane helped Ferguson win the Premier League seven times and the league and FA Cup double on three occasions. Keane dragged Manchester United to the 1999 European Cup final by force of his will, and thus gave Ferguson the platform to elevate his reputation to the level of his predecessor, Matt Busby.

Keane didn't get to play in that historic European final and Ferguson never repaid the favour of his protege by producing another team which looked remotely capable of becoming champions of Europe. At some point in every subsequent season Keane seemed to rant, Lear-like, upon the heath of United's failures as Ferguson bumbled around the transfer market looking for somebody who would be half-player, half-medicine bottle for the rest of his team.

Through the last few years those who watch Keane with fascination rather than horror have noticed several acts of metamorphosis. Keane himself - apparently nailing the weakness for drinking which once saw him greet the Manchester dawn in a police cell with Ferguson, avuncular and protective at that time, waiting outside - has become a Spartan in terms of his work-ethic and obsessive professionalism. It is said often that he hates losing, but it is more accurate to say that he identifies as losers those who fail to get 100 per cent out of themselves but are content with that. He hates losers.

The second metamorphosis has been in the character of Manchester United football club. Twelve years ago, when Keane left Nottingham Forest to move to Old Trafford, he was advised by those he respects to spurn the offer of better money at nouveau riche Blackburn Rovers and to opt instead for the solidity and values of Manchester United.

As advice goes that was something like asking a young man to open an account with the bank that likes to say yes rather than the bank that offers the free cd player: they'll both turn nasty when you're in overdraft. Yet United had the image, the tradition and the solidity which suited Keane. They had the manager too.

There were caveats. Always are. Football clubs are businesses, and business is business. Player beware.

The young Keane would certainly have realised all that in 1993, but the club he joined still existed as a somewhat old fashioned football club and they had a manager who exercised a dour paternalism which suited a player looking for a platform.

All changed. The gap between the Manchester United of the early 1990s and the Manchester United of the early 1960s is far smaller than the gap between the club he joined and the club he left. The club are now owned by the Glazers, a family of Americans who had never stood in Old Trafford before they owned the place and for whom the offside rule remains an impenetrable mystery. Manchester United are slaves to the stockmarket, they pander obsequiously to the immense Asian market. Winning big means fattening the shareholders.

Ferguson has changed too. Having announced his retirement a few years ago, and then foolishly deferring the date, he has been diminished in the dressingroom. His entanglement in the rather ugly Rock Of Gibraltar case shrunk his reputation still as he emerged looking like football's equivalent of a gal who does lapdances for the pleasure of richer men. Ferguson was the trophy-mate of rich men. He forgot that, in the end, business is business.

So Keane, changed, reformed and more focused than ever, emerged for the final stage of his career and found he was at Prawn Sandwich United and in cahoots with a manager who had developed notions about himself. That it should all have ended badly should surprise nobody.

On the other hand, almost everything about Keane surprises somebody.

His departure (the timing and circumstance of it) is sensational not because all Keane's departures have an absurd amount of high drama attached to them but because it highlights the essential callowness of soccer's soul. Manchester United may be the sentimentalists' favourite, the descendents of the Munich Disaster team, etc, but they have always been bottom-line merchants.

John Giles, the pre-eminent Irish midfielder of a previous generation, was let leave Old Trafford for £34,000. Decades later David Beckham left for Real Madrid. Football is about loyalty only when it suits football to be about loyalty.

This morning it is probably dawning on both Keane and Ferguson that they are (or have been) employees and working in the service of people who don't understand the perverted business model which is English soccer. Or perhaps the Glazers understand it all too well. It will also be dawning on Keane and Ferguson that there will be no winners from this.

Ferguson without Keane to enforce his message or spread his word looks stricken by the sort of cancer which courses through the body of a football man who loses his confidence. Surrounded in his castle by a retinue consisting mainly of overpaid mediocrities and answering to the call of a family of badly-dressed millionaires, Ferguson must bitterly regret not having passed gently into a retirement a few years ago when his stock was high and his legend was watertight.

After Saipan there was much philosophical debate as to whether it was truer to say that Roy Keane had been pushed or had walked. All that faff and cant didn't really matter. The biggest loser from the episode was Mick McCarthy, who met his end in a Lansdowne Road cauldron where the natives punctuated their booing with cries of Keano! Keano! So too shall it be with Alex Ferguson.

Keane is too proud a man to confess to something as human as feelings of wistfulness. He will miss out, though, on what seemed the natural ending to his career, the fleeting but telling appearances before the adoring faithful at United, the lucrative and heartwarming testimonial, the last goodbye.

It's not the way it should be and not, one imagines, the way Keane wanted it to be. When he announced a few weeks ago that this would be his last season at United, it seemed to many that he was playing his hand of cards and hoping to force Manchester United to play theirs. One more contract. A few years as the conscience of a restructured team.

As for the future, Glasgow seems to loom in most crystal balls. It is too late in Keane's professional life and family life for him to seriously consider a move abroad. He has declared himself unenamoured with the prospect of playing against United for another Premiership side. He is an impossibly wealthy man (who could afford, it seems, not to eat humble pie this year just because his testimonial was waiting at the end of it), and Celtic's limited circumstance shouldn't be too much of an issue.

So perhaps the greatest and most enigmatic of our modern breed of footballers will end his time as a player visiting places like Inverness on wet Tuesday nights, places not differing much in degree of glamour from those he started in as a teenager with Cobh Ramblers. Strange, but in the nature of the game. And wherever he is he'll be giving everything and raging against mediocrity.

In his nature, too.