Keith Duggan/Sideline Cut: Arsene Wenger's silence is, in the phrase of a famous Mancunian, louder than bombs. Every passing day seems to spin a fate more lurid and complex for the Frenchman's arch nemesis Alex Ferguson.

And Wenger, enjoying days of wine and roses at Highbury, has done exactly what one would expect of so calculating and calm a man. He has practically vanished.

It is hard to remember his last significant utterance or complaint. At times last season, he appeared to be in a state of perpetual disenchantment, scolding and strained, his Gallic sensibilities at odds with his English environment. The smoother his Arsenal team becomes, however, the more subtle his influence. That is the mark of breeding.

You can be certain that Wenger has trained that professorial expression on the invidious set of circumstances that are threatening to blight the autumn of Alex Ferguson's managerial career. So much of sport is down to karmic forces that, when the laws begin to turn against you, the deterioration can be swift and callous.

The remarkable sports story of the year to date has been that Ferguson, one of the key English football figures of the century, a man whose influence and savvy has been without parallel in the modern game, suddenly cannot put a foot right. For the first time since his perilous early months at United, the Scotsman looks impotent.

With highly-tuned ironic timing, the league schedule brings United to Loftus Road in Fulham this afternoon, the club Louis Saha effectively committed mutiny against to fulfil his ambition to join the brightest and the best. Saha's return in red colours will ensure a rowdy and hostile reception for United, an audible accompaniment to what must be a general feeling that the world is pitted against them.

The heavy-handed, unapologetic swagger with which Ferguson bought the impressionable Saha in both body and soul enhanced the perception that the United manager, and by extension the club he runs, felt somehow above - and better than - the insipid governance of the Premiership. The subsequent calls by the Cubic Expression shareholding group for investigations into many of United's major signings, however multi-layered the reasoning, placed an uncomfortable spotlight on the rigidity of Ferguson's football ethics.

And, with impeccable timing, ghostly reminders of Ferguson's key decisions have presented themselves at the very moment when he could have done with them staying locked away. Could Ruud van Nistelrooy have chosen a more inopportune week to wistfully declare that he missed the precise and gleaming crosses David Beckham had provided? Inevitably, the wilful sale of Beckham, who ranks with just a handful of athletes - Yao Ming, Ronaldo - for his global marketability - is proving a contentious issue.

From the beginning, selling Beckham to a Champions League rival like Real Madrid was an act of hubris. All the vapid criticism of the England captain during that transaction overlooked the essential fact that, for all his vanity and preening, he has a savage work ethic and was always going to be pushy and self-examining enough to exist among Real's blazing galaxy of egos. And he remains the most potent right winger born in England in the last 30 years.

Ferguson's searching auditions for a suitable contender for his former protégé's vacant position - Giggs, Solskjaer, Cristiano Rolando, Kleberson, Bellion, Scholes and Chris Eagles have all been played there - are tantamount to an admission that he has not mentally reconciled himself to Beckham's absence.

This issue comes hot on the heels of other stances taken by Ferguson now being interpreted as lax. The decision not to replace the monumentally irresponsible Rio Ferdinand led indirectly to Wednesday night's preposterous Neville-Neville combination among United's fragile back four in Porto. The charge is that the Ferguson of yesterday would never have been so complacent; indeed, under the Ferguson of yesterday, Ferdinand would not have dared to so vacuously compromise his club's season with his amateurish behaviour.

The jettisoning of senior men like Laurent Blanc and Seba Veron, the cavalier treatment that is all but forcing Nicky Butt to leave his boyhood club and the unproven purchases of Ronaldo and Kleberson all add fuel to the fire of those who wish to portray Ferguson as vulnerable.

And the final, exquisite sufferance has come through Roy Keane's needless grace note when the day in Porto was already done, just when the comments of both Keane and Ferguson regarding responsibility and attitude at the club were still ringing sharply in the ears of the younger players. Ferguson's fiery and unhappy exchange with his Porto counterpart was significant. In a melodramatic mood, you could liken him to Captain Bligh in the weeks before the revolt.

The disintegration of sporting dynasties is much more fascinating than their life span. There is nothing more boring than a team habitually winning against inferior opposition, as United did for so long. There are numerous instances of great sports teams that seem to have the formula for greatness bottled, only to imperceptibly and irrevocably lose it. Red Auerbach is little known here, but the former manager of the Boston Celtics was cut of the same cloth as Shankly and Busby. As the last great Celtics side broke down, Auerbach, by now just an upstairs figurehead - like Busby in his senescence - was asked to comment on a classic play-off game the Celtics had just lost. It was the last game to be played in the old Boston Garden, scene of Auerbach's many classic triumphs, and he was asked for a lyrical sound bite to encapsulate the nostalgia of the occasion.

"F*** the building," the old man snapped. "We just lost a ball game." That was in 1994 and the Celtics have largely been an irrelevance ever since.

The power base in English soccer is too narrow and loaded for Manchester United to slide to that degree. But money alone does not buy success - or at least the standards of success Ferguson deemed acceptable for the club. There is the real sense now that all of Ferguson's noisy triumphs hang in the balance and that he is teetering on the edge of the crevasse. That is why this Wednesday evening, a goal down to Porto and Keane disconsolate in the stands, is such a significant evening at Old Trafford. It is the kind of challenge Ferguson once lived for.

It is not that Porto represent such a terrific challenge, but rather that the odds and the gods seem stacked against United and Ferguson these days, perhaps for no better reason than, in sport, what goes around always comes around.

Whether he still has the energy for such naked confrontation with these elemental forces and whether he can point an increasingly disparate collection of players in the same direction is the true drama.

The feeling here is that Ferguson and United will ride out the storm seas and have a greater say on this season than is reasonably probable at this stage. We will know United are back if we hear the voice of Arsene any time soon.