But newspaper reports following the deal claim that the delay was United’s doing – because they refused to meet Chelsea in person to discuss the transfer.
Mata was set to arrive at United’s Carrington training ground complex by helicopter on Thursday. United were so convinced that they set-up a temporary helipad in anticipation of the ex-Valencia playmaker’s arrival.
But Mata never arrived, left in limbo – it was presumed – because Chelsea wanted to confirm Basel winger Salah first.
However, while Mata’s signing for United on Saturday came around the same time that Salah agreed terms with Chelsea, the Egyptian winger is yet to be awarded a work permit, meaning – technically – he is not eligible to represent the Blues in a competitive match.
Salah, 21, is almost 100% guaranteed that piece of paperwork as he is Egypt’s star player and thus fulfils the criteria of having played 70% of their international matches in the past two years.
Still, if Chelsea were concerned about sorting that deal first, why did Mata complete his registration before Salah is confirmed?
Because, it transpired, that was not the real reason for the delay - as explained in this article in Monday's Guardian newspaper.
Mata, 25, had been Chelsea’s player of the season for consecutive years before finding himself out of the first-team picture following Jose Mourinho’s summer return to the club.
Chelsea were happy to let Mata leave for the right price, and verbally agreed a £37 million buy-out clause at the start of the transfer window.
Initially this clause excluded United, but after meeting with Mata’s father – also called Juan – the club agreed to let the Spaniard move to Old Trafford, despite David Moyes’s team theoretically being one of Chelsea’s biggest rivals.
United chief executive Ed Woodward, who had made initial inquiries about the Spaniard last summer, was so nervous about Chelsea trying to open talks over England striker Wayne Rooney that he refused to meet the Blues and instead instructed his staff to communicate entirely in writing.
Chelsea were keen to meet United directly – one presumes because they were keen to see if a deal for Rooney could be completed in tandem with the Mata sale – but United instead issued a formal letter informing that they were prepared to meet the £37m clause, and to pay the fee in three instalments as requested.
That prompted United to prepare for Mata’s arrival – but Chelsea responded by asking for face-to-face talks, which United again refused.
Meanwhile, agents representing the player and both clubs worked behind the scenes to find a compromise. Finally Chelsea yielded as Mourinho told the press he would let Mata decide his future, paving the way for the transfer.
All this may seem a bit rude and aloof, but you can see why Woodward decided to keep his distance.
In the summer United – and Woodward in particular – came under criticism for failing to play the transfer window as expected. Keen to bring in top talent following the retirement of Sir Alex Ferguson, United attempted to sign a host of transfer targets, but were apparently strung along by agents and representatives of selling clubs.
In the end they overpaid for Marouane Fellaini, failed to snare the likes of Cesc Fabregas and Ander Herrera because of mixed messages from their advisors and clubs, and ended up missing out on Mata because Chelsea kept bringing up the topic of Rooney, who Mourinho remains keen to sign.
As a result you can hardly blame rookie chief exec Woodward for playing things safe – Fellaini taught him that, if a release clause exists, he should meet it in writing immediately; Fabregas and Ander taught him to treat agent claims about player interest and buyout fees with caution; Mata taught him that – if you are prepared to meet a fixed fee for a player – just send an email to avoid complicating matters.
While this style of transfer business is unusual in England - where coaches, players, agents and directors normally sit around a table to thrash out a deal - it is the standard in many European countries.
In Germany and Spain, for example, it is a legal requirement for players to have buyout clauses in their contracts. Sometimes these clauses are prohibitively high, but otherwise the likes of Bayern Munich will often do business by sending a fax and transferring the transfer fee into the selling club’s account.
Having been burned last summer, it seems United are happy to follow this lead when they can. Certainly in this case it allowed them to protect Rooney, who is now expected to sign a £300,000 per week contract at the club.