It seems barely credible to think that in a football season that takes in 380 Premier League matches, involving more than 400 different players and countless debatable refereeing decisions, so much can hinge on a private conversation between two men that took place before a ball was kicked.

Back in August, Sir Alex Ferguson picked up the phone and dialled Arsène Wenger's number. It was a pivotal moment. Both managers agree on that, and so, too, does Roberto Mancini. That dialogue provided the revs needed for United to go the "extra mile" that Ferguson deemed necessary to swing the deal for Robin van Persie. The two men talked. Conversations of this nature are not always entirely comfortable, especially in this particular situation. One of them was in the business of helping himself while weakening the other. And both knew it. Nobody could pull the wool over anyone's eyes over the significance of this deal.

Several months down the line, Wenger pauses for a moment as he recalls the tone of the exchange, searching for the right word. "Professional," is the one he plumps for, although the hint of a knowing smile suggests the negotiations may have been infused with a touch more emotion. "It was a very professional phone call, and there was more than one," he adds, arching an eyebrow. "I will tell you the rest another day." That is quite the teaser.

Ferguson felt compelled to pull out all the stops because Van Persie ticked every box on his wish list. As a footballer, the evidence had been overwhelming during a season when he was crowned Footballer of the Year. As a professional, Van Persie's behaviour in recent years made him much loved and admired by those with whom he worked. Although his advancing age meant United had to break their usual pattern for signings, with players young enough to have a resale value the norm these days, his experience was also a glowing attraction.

With injuries limiting the game time of Rio Ferdinand and Nemanja Vidic, and age restricting the use of Ryan Giggs and Paul Scholes, Ferguson wanted to inject some more authority into the team. It is far too early for the comparisons with Eric Cantona to be meaningful, but what Van Persie has achieved in his first few months at United is an aura. His imprint on the team is already in bold. There is a feeling that he can be the hub, the inspiration, the man that can dig United out of a hole, catapault them upwards, with a metaphorical click of the fingers. It is no coincidence that the name most often and most lustily sung by the Old Trafford crowd, other than Cantona's, is his.

In assessing Van Persie's influence on United so far, it is telling that he has struck a mighty chord with both his ability and his personality on the pitch. Van Persie is able to carry the sense of expectation around him. He wears it very well, in fact. It stems from the fact he feels so in tune with his football, and he possesses an interesting blend of being both incredibly determined and incredibly relaxed when he is trying to shape a game.

Wenger sounded ever so slightly nostalgic as he reflected on how everything was in place for Van Persie to prosper right from the start at United. "What he has is a high level of confidence in his qualities. He knows his game well," his former manager said. "He knows how to exploit his qualities and he is more patient than he was five or six years ago. Now he stays up front. You maybe won't see him for 20 minutes. But he knows he will get a chance and use it. Before he used to come back into midfield. He uses his killer instinct much better now. He is at the peak of his game, 29 going on 30. He uses all his qualities in a calm and intelligent way."

That has manifested itself in the way he has changed club and yet, without breaking stride, has continued leading his team from the front, making the difference, scoring goal after beautiful goal, including the winner in September against Sunday's opponents Liverpool. On 13 occasions his strikes for United have had a direct impact on the result. As Ferguson plainly puts it, "He has become really important to us."

A few years ago, when he was beginning to emerge from a series of seasons where his development was hindered by injuries, Van Persie gave an insight into how he feels when he is immersed in a close, tense, contest. It's a perfect description of the matchwinner he has evolved into: "I live on having a challenge when the line is really thin," he explained. "I like it the most when you have no clue what is going to happen. Everyone is on top of their toes, totally excited. These situations get the maximum out of me." That was crystalised on numerous occasions this season, most recently last week at West Ham when his work of art in stoppage time turned the game on its head.

It has not taken Ferguson long to get to grips with what makes the Dutchman tick. That's not surprising, because the Scot will not have come across many players more obsessed by the game, more willing to discuss tactical nuances, more knowledgeable about different teams and players and ideas, than Van Persie. They clearly get along just fine and have had some nice get-to-know-you chats. "It certainly wasn't about the weather in Holland or England," Ferguson says. "Just decent general conversations. I've done that with a lot of players who come to the club. You want to get closer to them and find out who and what they are in terms of character what their likes and dislikes are and how they look upon football. And in some ways what they think tactically about the game."

Naturally Ferguson could not be happier that his £24m man is reaping such rewards. "Any player coming here for the first year, you never know for sure," he said. But with Van Persie, the manager's level of confidence was unusually high. "You never know for certain what you're going to get but what we did think was that we're signing a very good player. His background reports have always been good. It was a lot of money but you either want him or you don't, and you have to pay the price that the club is sticking out for. We wanted him badly."

The fact Manchester City also had their eyes on him adds piquancy, with Mancini's assertion that Van Persie is the difference this season hard to dispute at the moment. "I wasn't aware of the City thing until he told me himself that he'd turned them down," Ferguson said. "I did know that Juventus had made a massive offer to the boy and I thought they were a bigger threat, but fortunately he came to us."

Sunday's game against Liverpool includes the chance to see the Premier League's front-running forwards, with Luis Suarez in opposition, but that is, according to Ferguson, the kind of fish that is so small he wouldn't bother to look for his fryer: "They are both good players. I can't analyse them in terms of differences and likennesses, I haven't got time for that."

Wenger says he does not find it painful to watch Van Persie delivering, decisively, game after game in a different shade of red. "But what is painful is to see United so far in front of us," he says. "We knew when we sold him to United that would be the case." That is a stark admission from Arsenal's perspective. From a United point of view, when he picked up that phone, Ferguson sure knew what he was doing.