HISTORY tells us that no matter how big and powerful they are, empires don't last forever. We all know what fate befell the "invincible" Holy Roman Empire. As recently as 100 years ago the arms of the British Empire stretched right round the world. Now those days are just a fading memory. And so it is in the world of professional football. Empires come and go. Leeds United under Don Revie in the early 70s. Liverpool under Bill Shankly and then Bob Paisley in the 70s and 80s. But one empire has bucked the trend and defied the history books. It is, of course, the empire built by Sir Alex Ferguson at Manchester United. Yes, during Fergie's tenure at Old Trafford, other clubs like Arsenal and Chelsea have briefly stolen United's thunder. But the fact remains that for the best part of two decades the Old Trafford empire has dominated English football. That remarkable achievement speaks volumes for the management skills of Ferguson who has demonstrated down the years an amazing ability to dismantle a great team and build a new one equally great or even greater. But there are signs now, I believe, that the magnificent empire built by Ferguson is finally starting to crumble. And that theory has nothing to do with the one-off FA Cup defeat at the hands of Leeds United. As I've just mentioned, Fergie in the past has always replaced great with great. It took him a while but he found a superb replacement for Peter Schmeichel in Edwin van der Sar. He lost a natural goal-scorer when Ruud van Nistelrooy departed, but found a better one in Cristiano Ronaldo. On the strength of Dimitar Berbatov's performances for Spurs he looked an ideal signing. But quite clearly that hasn't proved to be the case. Berbatov has got sublime skills, but his languid style doesn't suit United's style of play - and I suspect it never will. But the cracks are showing elsewhere in Ferguson's team. Compare Anderson and Nani to Paul Scholes and Ryan Giggs at the same age. There is no comparison. Two of them were world-class stars at a young age, the other two are pale imitations. Scholes, Giggs and Gary Neville can't go on forever. But if there are adequate replacements waiting to take over from those three great players in Ferguson's current squad then I've failed to spot them. Could it be that Ferguson simply hasn't got the cash at his disposal to buy the players he needs to ensure that his club's dominance continues? I have no answer to that question. What we do know is that United received £80m from Real Madrid for Ronaldo. Has Ferguson got access to that fortune? If he has it's baffling that he's spent so little during last summer's transfer window. What sticks out like a sore thumb is that, despite their current league position, United this season have looked a shadow of the team which won consecutive Premier League titles in the previous two years. Based on the displays I've watched during the first half of the season I don't believe United are capable of winning either the Premier League title or the Champions League. Despite the idiotic muttering of a few disgruntled United fans Ferguson is not on the wane - but his team is. In my book he needs another right-back, two quality midfielders and a mobile 25-goals-a-season striker. If the club's American owners cough up the money for Ferguson to recruit players like that during the current transfer window then the Old Trafford empire will remain unchallenged. If they don't, that empire is in serious danger of collapsing and a new emperor will be occupying Sir Alex Ferguson's throne.