The notion of a parallel universe is one that most of us struggle to comprehend, but in football, the concept of alternate realities livens up many a bar-room debate. What if that Soviet linesman at Wembley in 1966 had decided that Geoff Hurst’s shot did not cross the line? What if Manchester United’s board had lost its nerve in January 1990 and brought Alex Ferguson’s reign at Old Trafford to an ignominious conclusion? And what if a Russian billionaire had not pitched up as the saviour of a financially stricken Chelsea in the summer of 2003? How different would the football world be?

So here is another alternate reality for supporters of Liverpool and Manchester United to ponder: Fernando Torres and Cristiano Ronaldo lining up on opposing sides at Old Trafford tomorrow lunchtime, but with Torres in the red shirt of United and Ronaldo in the white change strip of Liverpool.

It could very easily have happened. United’s interest in Torres was well documented during the years before his £20.5 million move from Atlético Madrid to Liverpool, with Ferguson holding what were believed at the time to be productive talks with the player’s advisers in Paris during the summer of 2006.

Less well known is just how close Ronaldo came to moving to Anfield three years earlier, when Gérard Houllier was offered the winger, 18 at the time, for a fee of £4 million. It seemed exorbitant for a raw teenager, but within months, Ronaldo was on his way to Old Trafford for three times that sum and, almost five years on, he is valued by United at more than £50 million — and, more pertinently, as irreplaceable.

The story is recounted by a typically candid Phil Thompson, then Houllier’s assistant manager, in his recent autobiography, Stand Up Pinocchio, and, although the title might sound like a work of fiction, Liverpool supporters should be warned that every depressing word is true. “Yes, he was good,” Thompson wrote. “Portugal had two starlets, [Ricardo] Quaresma and Ronaldo, who played right and left wing for their under-21s. I watched them play and both were very good. It was a toss-up as to who was the best.

“I was invited to watch Sporting play Porto in the last game of their season. Tony Henry, the former Manchester City player and an agent with Paul Stretford’s ProActive agency, was on the phone on a regular basis to see if we would take Ronaldo and asking if he could take us to watch him. I met Tony at the airport and travelled to Oporto. I met the player’s Portuguese agent before watching the game.

“Ronaldo was quite good, but not as impressive as the first time I saw him. Tony was pushing the boy and saying he was a talent. He was saying, ‘He’ll only cost £4 million. It can be paid over the course of his contract at

£1 million a year.’ He also said the player wanted £1 million [a year] tax free. I said, ‘The boy is only 18. That is a massive problem.’ We would have had anarchy if the other players had found out how much we were considering paying for an 18-year-old kid in Ronaldo.”

Yet Liverpool were giving serious consideration to the idea when they heard shortly afterwards that Ronaldo was on his way to Old Trafford, having run United’s defenders ragged while playing for Sporting in a friendly match that week. Contrary to popular myth, United had already agreed a deal to sign him before Gary Neville and others urged Ferguson to do so that night, but Ronaldo’s performance — and their heightened determination to seal the deal before they left Lisbon — drove up the fee to a mouthwatering £12.24 million.

It is a fee that Thompson describes as “astonishing” and it is worth recalling that John Magnier and J. P. McManus, then the club’s two largest shareholders, asked the United board some serious questions at the time about that and various other transfers. However inflated it might have seemed at the time when United were desperate to find a player to replace the departed David Beckham, it is a deal of which they are justifiably proud.

And so the question raises itself: how would Ronaldo have developed at Liverpool, as opposed to United? There is a strong case for saying that a player of his technical ability was always destined to be a success, but think back to the player he was at the time: hugely talented, yes, but erratic, selfish and at times absurdly misguided.

Think also of the Liverpool of 2003-04. Could Houllier, in what transpired to be a difficult final season in charge, have afforded to indulge such a raw teenager, however talented?

The chastening experiences of Anthony Le Tallec and Florent Sinama Pongolle, the French protégés whom he signed that summer, suggest not. At the time, having excelled in numerous youth tournaments, Le Tallec and Sinama Pongolle were rated, alongside Ronaldo, among the most promising talents in the world, but now they play for Le Mans and Recreativo Huelva respectively. Would Ronaldo have gone the same way? Probably not, because he appears to be made of stronger stuff than Le Tallec, in particular, but it is feasible.

Another purely hypothetical question: if Ronaldo had developed as successfully at Anfield as he has at United, could Liverpool have kept hold of him? Could the team have satisfied his ambitions and could the club, in their precarious financial state of recent years, have resisted the money on offer from Real Madrid? Could they have persuaded him, as they did Steven Gerrard after some difficulty, that playing for Liverpool has a value far beyond that of competing at the top end of the Premier League? Or would the arrival — and development — of Ronaldo have turned Liverpool into champions? Questions, questions.

What is certain is that, finally, Liverpool have signed a player to rival the box-office appeal and match-winning quality of Ronaldo. Torres has been little short of phenomenal in his first season at Anfield, scoring 27 goals in all competitions and giving Rafael Benítez’s team the kind of cutting edge that had been sorely lacking.

It is tempting to wonder what Ferguson makes of the Torres phenomenon. He had a longstanding interest when the forward was at Atlético Madrid and, after a couple of near-misses, finally came close to signing him during the summer of 2006.

After the aforementioned meeting in Paris, United made a firm inquiry to Atlético, but, having previously been encouraged, were told that the forward wanted to stay in Madrid and would not be sold for less than the £40 million fee stipulated in his contract. United dropped their interest, deciding to focus instead on their midfield, and, to the surprise of just about everyone, went on to win the Premier League title that season.

United had the opportunity to renew their interest in Torres last summer, but by this stage their fervour had faded, partly because they had concluded that he would never come to England and partly because Ferguson and his staff believed that he lacked composure in front of goal.

That is a verdict that now seems laughable, given the sublime finishing Torres has shown in recent weeks, but even those close to the forward admit to being pleasantly surprised by his transformation into a natural goalscorer since his move to England, where prolific strikers from La Liga or Serie A have usually endured the opposite experience.

Carlos Queiroz, the United assistant manager, recently said that he was “very impressed” by Torres’s form at Anfield. “He sees spaces that others can’t and he has the ability to penetrate these areas with or without the ball,” Queiroz said. In other words, he is the type of centre forward that United, for all the qualities of Wayne Rooney and Carlos Tévez, do not have. Ferguson was less gushing on the matter yesterday, but then it is not his style to wax lyrical about opposition players. “I don’t think he came here with any great doubts,” the United manager said. “The boy had a great record in Spain. But, to be honest, I haven’t paid that much attention to how he was going to settle in because he’s not my player. Having said that, when we play a team, any team, we have to prepare properly, looking at the assets of our opponents and trying to prepare ourselves the best we can.”

For Ferguson, that means formulating a game plan to neutralise the threat of Torres. For Benítez, it means trying to find a way to stop Ronaldo. Somewhere in a parallel universe, they are faced with the opposite problems. But it says everything about the tribal loyalties of football folk that neither Liverpool nor United would swap their man for the other now, even if, in quieter moments, they do privately rue the one they allowed to get away.