It often comes across as churlish to talk about the relatively minor errors made my managerial masters, but amidst the raft of excellence, mistakes will inevitably crop up even for the best in the business.

Pep Guardiola’s magnificent run with Barcelona included the now-infamous €25 million signing of centre-back Dmytro Chygrynskiy as well as misguided moves for Aleksandr Hleb and Martin Caceres worth a total of €36m.

He also managed to make a €53m loss on Zlatan Ibrahimovic in the space of 12 months.

For Arsene Wenger at Arsenal there is a Kaba Diawara for every Thierry Henry, while Jose Mourinho didn’t only sign Didier Drogba, Ricardo Carvalho and Michael Essien for Chelsea… He was also responsible for additions such as Asier del Horno and Maniche as well as the sale of Kevin de Bruyne.

Even Sir Alex Ferguson is not untouchable, with many considering the purchases of players such as Eric Djemba-Djemba, William Prunier, Massimo Taibi and, most pointedly, Bebe as examples that the Scot was as fallible as any other manager in the market.

And while his transfer record undoubtedly goes down in the annals as a very positive one overall, there can be few moves in the history of the game which reflect so badly on a decision than Paul Pogba’s return to Manchester United does on Ferguson’s choice to let him leave Old Trafford four years ago.

It was fairly clear within a few months of Pogba arriving at Juventus that United would be left kicking themselves, but the true cost of Ferguson’s belligerence over the French midfielder has only been quantified this summer in the way of his £93.8m move back to Manchester.

Ferguson spoke of the difficulties with Pogba and his agent Mino Raiola in his 2015 book ‘Leading’, explaining: “There are one or two football agents I simply do not like, and Mino Raiola is one of them. We had Paul under a three-year contract, and it had a one-year renewal option which we were eager to sign. Raiola suddenly appeared on the scene and our first meeting was a fiasco. He and I were like oil and water.

“From then our goose was cooked because Raiola had been able to ingratiate himself with Paul and his family and the player signed with Juventus.”

As tensions rose towards the end of their spell together, Ferguson overlooked Pogba for a potential first start at senior level and there was no turning back in the mind of the player.

“It was a very, very difficult moment for me because I was in love with Manchester and I was a Mancunian,” Pogba told Canal+ in 2014.

“It was the match against Blackburn in December 2011 at Old Trafford.

“Paul Scholes had retired, Darren Fletcher was injured. There was no one left to play in midfield. And I was training and I was beginning to get better bit by bit, and the coach never stopped telling me: ‘You’re this far’.

“And I didn’t understand. This far away from what? Playing? From having some playing time? From getting on the field? Or what?

“And there was Rafael in midfield and I was disgusted. I was disgusted and I didn’t get on either.”

In failing to make a decision based entirely on its merits – nobody could claim Rafael was truly a better option in midfield than Pogba – Ferguson effectively forced out a player who within four years has cost United a fee no club has ever paid before.

It is fair to class it as a costly error.

Ferguson would doubtless still tell you that he made the right choice. As a man with a very distinct set of principles he was not one to budge when he had a decision in mind, and that ended United careers for far more high-profile players than was Pogba at the time.

The likes of Roy Keane, David Beckham and Jaap Stam were all shown the door by Ferguson to the surprise of many onlookers.

But the sheer weight of the sack of gold which arrived at Juventus Stadium on Monday evening tells its own story with Pogba.

The 23-year-old has learned an awful lot about football since heading to Italy and there are some things he has picked up in the black and white which he could never have added to his game as a United player.

Yet even that cannot explain away the £93m loss made on a player for whom Ferguson received £800,000 in compensation back in 2012.

The saving grace is that Fergie’s was not an irreversible error, with Pogba’s return proving that ties were not severed beyond repair. But it might never be surpassed as the most expensive out-of-contract departure in the history of the game, and one which will leave a question mark over the legacy of football's greatest-ever manager.