Perhaps the most intriguing part of the long, complex process that finally led to a helicopter carrying Juan Mata arriving at Manchester United's training ground on Saturday is that a £37.1m deal can be arranged without as much as a telephone call between the clubs. Several months of positioning, mutual suspicion and political bargaining, and everything was done without a single word being exchanged in person.

Chelsea certainly tried. On more than one occasion, a message reached Ed Woodward, United's chief executive, that the relevant people at Stamford Bridge were open to sitting at the other side of the negotiating table. Each time, he politely declined. Everybody in football knows Chelsea want to sign Wayne Rooney and Woodward reasoned that it would be virtually impossible to keep them sweet on Mata while also informing them they could forget about anything happening the other way.

Instead, the background to the Mata deal is a case study about how complicated and sensitive these big-money arrangements can be, and an insight into the high influence of agents in the modern business. Without them, both clubs recognise the chances of the deal going through were nonexistent. Seven different agents approached United since August to say they could make it happen. As it turned out, they used Mata's father, Juan senior, and Colin Pomford, a Madrid-based agent who specialises in Spanish business going back to Steve McManaman's time at Liverpool and, later, the Rafael Benítez era.

What has never come out before is that United first heard Mata was open to leaving Chelsea, and keen on moving to Manchester, late in the August transfer window, after he had picked up strong vibes from José Mourinho that he would be used only sparingly. Woodward never actively followed it up because of the Rooney situation. He did, however, establish that David Moyes liked the player.

Their next information – after the window closed – was that Mata was so unhappy he had been to see the top people at Chelsea, among them Roman Abramovich, and been given a verbal agreement that if he carried on being left out of the team, and maintained his professionalism, he would be allowed to leave in January. Chelsea would later harden that into a written agreement that included a €45m buyout and – possibly still thinking about the Rooney issue – one proviso: that the figure could not be activated by United.

That could easily have been the end of the deal. Except Mata and his father went back to Stamford Bridge, once it became clear there was interest from Old Trafford, and asked for the condition to be removed. One of the people involved in the negotiations has told this newspaper that Mata and Abramovich are "almost like friends". Mata had kept to his side, bar one fit of pique after being substituted, and Abramovich respected that.

Chelsea gave him a verbal promise, but nothing in writing, and then sent a message, via Pomford, that maybe it was time the two clubs met face to face. Woodward said no. He simply did not want to risk changing the temperature by getting involved in any talks about Rooney and, with nothing in writing, he was not willing to take any chances about the price suddenly going up.

A separate message came back to United that Chelsea would welcome an official bid on a specific day. Woodward declined again. Instead, his lawyers faxed through a "transfer agreement" informing Chelsea that United had met the buy-out clause and would pay the money, as stipulated, in three parts.

But the game of cat and mouse continued. Chelsea indicated again they wanted to talk and would even let Mata travel to Manchester to take his medical. Except Woodward knew, again, that Rooney would crop up and potentially kibosh the whole deal. "He couldn't risk it," according to one source. So he sat tight, while Mata and the relevant agents went to work behind the scenes. Then Mourinho went public that Mata could leave and that was always going to speed up the process. Finally, the deal was closed on Saturday.

For Woodward, it is the biggest coup of his time in charge at Old Trafford. United have been accused of panic-buying and maybe, in light of the new detail, that is an unfair slant to put on it. The club had, however, advised journalists to "ignore" any speculation about Mata earlier in January, and sources say they still felt as recently as Tuesday that it was far more likely to happen, if at all, in the summer.

If Mata still feels like a strange buy, it is largely because Moyes has insisted for several months that all he wants is a central midfielder and a left-back. Questions about Mata were knocked back on the basis United already had Wayne Rooney and Shinji Kagawa for the No10 position, in line with the club's explanation for not challenging Arsenal for the signature of Mesut Özil.

So why Mata and not Özil? Timing, basically. Özil was offered to United in the first week of the summer transfer window, but Moyes had barely seen Kagawa at that point and wanted to learn more about a player about whom Sir Alex Ferguson had left glowing reports. As it has turned out, Kagawa has been disappointing. United, however, would prefer to keep him for now, not least because Mata is ineligible in the Champions League.