Comment: Could Ferguson be on a downward spiral?

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Sport, as Lance Armstrong would appreciate more than most, goes in cycles.

No-one stays at the top forever. Not Australia's cricketers, who looked unassailable for the best part of two decades but who start another Ashes series next week appearing distinctly vulnerable.

Not Roger Federer, who lost his world number one ranking last year, even if the bookies still have him favourite to win another Wimbledon crown on Sunday despite the challenge of Andy Murray.

Not even it seems Sir Alex Ferguson and Manchester United.

If you had to choose any sporting monolith which appeared to be set in stone then it would have been United, with its rich history, its global fan base and its legendary manager.

But this summer there are signs that, if the foundations are not exactly shaking, a crack or two are appearing.

One such fissure came this week with the news that Karim Benzema, a striker widely reported to be Ferguson's top transfer target, had chosen to leave Lyon and join Real Madrid to team up with Kaka and Cristiano Ronaldo instead.

It came the day Ronaldo's move to Real was formally concluded and as Bayern Munich's Franck Ribery was telling the world he wanted to leave, but only for Real.

So in the space of a few weeks United have sold their greatest asset in Ronaldo, failed to persuade Carlos Tevez to stay, missed out on their number one replacement, Benzema, and been snubbed by Europe's most wanted midfielder.

But they have signed Antonio Valencia, you might argue, for around £16m.

Which is true, but that is like saying you have sold a five bathroom mansion on Sandbanks and bought a three bed semi in Bournemouth.

Look at the figures.

Ronaldo scored 68 goals for United in the last two seasons. Benzema scored 54 for Lyon. Kaka weighed in with 35 for AC Milan and Ribery 33 for Bayern.

Valencia? In the three seasons he spent with Wigan, since first joining on loan in 2006, he scored seven goals. He is a pleasing little winger with good balance and a decent delivery but for £16m you might expect a shade more evidence of fire power.

Which is why Ferguson continues to scour the football world for the striker he needs to replace Ronaldo and augment the remaining two from the so-called 'Fab Four' - Wayne Rooney and Dimitar Berbatov. Ferguson's problems start there, but they do not end there.

Because while Ferguson has young guns such as Federico Macheda and Danny Welbeck knocking on his door for more opportunities he also has the impending retirements of Ryan Giggs, Paul Scholes, Edwin van der Sar and Gary Neville to contend with.

And he has to do so at a time when the Premier League might well be losing some of its financial lustre for the biggest stars.

Already Arsenal's Andrey Arshavin has complained about having to pay 40% tax on his earnings and that rate rises to 50% from next April.

The pound is weak, the Euro is strong and television channel Setanta has gone into administration because it could not lure enough subscribers to watch its football packages.

Meanwhile, Italian football is emerging from a decade of decay and Spain appears once more the place to be.

Cue the job Josep Guardiola is doing at Barcelona with stars such as Lionel Messi, Xavi, Thierry Henry and Andres Iniesta.

Just five weeks ago we watched and admired the Spaniards outclass United in Rome's Olympic stadium to wrench the Champions League trophy from the grasp of Ferguson.

It might have been a blip in football's balance of power and Ferguson, as critics know to their cost, should not be written off. But, as players go and others fail to arrive, it is beginning to have the feel of a new cycle.