It's been a disappointing season for the Red Devils in many ways, but the ramifications of Pogba's decision are deeply worrying.
Although there is as yet no news as to why the young Frenchman has made this decision, it can only be seen as a rejection of Sir Alex Ferguson's personal wish and belief; and a doubt about where Pogba will be best placed for his future development.
Let's be clear here. Pogba has been regarded as one of the best young prospects of his generation. People have compared him to a young Patrick Vieira.
His form, particularly last season and this for the Reserves and Academy has been sparkling, including stunning strikes and crucial goals.
He can do everything. At 6'1", he is a sound header of the ball, is useful in both penalty boxes in the air, has a great shot in both feet and can ghost past players. His future was assured at United.
Sir Alex Ferguson has already marked him out as a future star: “He’s going to be a first-team footballer at Manchester United there’s no question."
To say I'm stunned at this news, however, is an understatement. Its bad enough that United look likely to miss out on the Premier League title, having gained only four points out of 12 before Sunday's win against Swansea.
There have also been many other disappointments, of which you are all aware, that don't need to be reiterated here. Ravel Morrison was not one of them, except for the fact that he is likely to waste his talent through arrogance and indiscipline.
The problem about losing Pogba is the messages it sends.
Of course Sir Alex only wants players who want to play for the shirt and stay for the rest of their career. He may not have issued an ultimatum to Pogba, but he will also not have allowed himself to be held hostage.
These, however, are some of the many questions supporters will want answering, because they have ramifications for other young players coming through the ranks, as well as for prospects United want to sign in the transfer market.
How did United let him get away?
It is clear that Juventus have been chasing Pogba for some time. We don't want to reopen the debate over how United got him from Le Havre in the first place; that has been put to bed. But when young players travel to Europe regularly these days—Pogba is the French U19 captain—there are plenty of opportunities to get to them while away on international duty.
Mino Raiola is also regarded as one of Europe's "super-agents." He represents Mario Balotelli, for example. He not only will get a substantial fee from Juventus, which United may not have been prepared to match, but he has strong contacts in Italy.
It seems United were prepared to match Juventus's rumoured £800,000 a year offer, which would have been a major step for a player who is nowhere near established in the first-team squad, let alone the 18 or the team.
While that gives an indication of the value placed on him, you get the sense that this has come too late and has been offered reluctantly.
From Pogba's point of view, however, there is the question: Why he wasn't more established?
Pogba is not a late developer
It somehow seems preposterous that United paid £16.5 million for Phil Jones, on a contract that may well be at least £45,000 a week at the age of 19, but £20,000 a week had to be wrung out of a stone for Pogba, supposedly the brightest young talent of a generation.
Why is it so hard to break through at United when they have so much young talent? After all, the "Fergie's kids" generation were of a similar age and went on to win a treble.
Was Sir Alex desperate in 1995 when he blooded five young players all at once? Is that the master plan next season? If that's the case, the centrepiece has just fallen out.
I've watched MUTV for nearly 20 years. I've seen every bright young talent bud and bloom or fall by the wayside. Ravel Morrison was equally as precocious as Pogba, but his criminal record and indiscipline at the club finally broke Fergie's patience.
Even at 15, Pogba was a star in the making. For the last two or three years at least, it has been clear that United had no replacement for Paul Scholes. Instead of trying fruitlessly to sign the likes of Luka Modric, Wesley Sneijder and Samir Nasri, why wasn't Pogba fast-tracked into the first team squad, playing against lower lights in the League, rather than sporadic appearances in the Cup?
Wouldn't it seem somehow ironic if United paid £20m-plus for Eden Hazard or Christian Eriksen, who, in my opinion, will not be as good as Pogba in five years?
Why was Sir Alex prepared to pay almost £26 million for Wayne Rooney at barely 18 and not bust the bank for Pogba?
Ah, you say, Rooney—and indeed Jones, Hazard and Eriksen were all established players, whereas Pogba has barely featured for the first team and not even started a match.
What does it say about United's future?
Sir Alex worries about whether he will be able to compete with Manchester City in the transfer market. Not much more than a year ago, Wayne Rooney was ready to "up sticks" and move (it was rumoured) to United's "noisy neighbours."
Why? Because he felt the club had lacked ambition, especially in not signing the sort of quality players needed to maintain their preeminence and fight off the challenge of the "blue mooners."
Why would Rooney even consider going to the Etihad (if true). Having more money is a help—especially when it means that no player is beyond your pocket. But they still have to be blended into a team and no one is better than Sir Alex at that.
Or could it be that Wayne could see the day approaching when Sir Alex was no longer around and concerned about the uncertainty that might bring?
Certainly Pogba could not count on Sir Alex shepherding his development to greatness for more than a few years at most. The Scottish knight isn't going to be around at 80.
But choosing to go to Juventus makes a very big statement about Pogba's view of Manchester United. While Serie A is littered with financial basket cases just like La Liga, Juve are on the way back to greatness.
They are one of the most successful football clubs of all time—the most successful in Italy—and have just won the League.
United qualify on all those counts except that almost certainly they have just lost the League to a club with more money.
At least Pogba has had the decency not to cross the City. Maybe he realised that his welfare might be at risk, but where he could have joined another outrageously precocious Raiola player, Balotelli.
Who knows, after this season's shenanigans, the two of them may yet end up playing together next season at Juventus.
So, arguably one of the greatest talents United have ever developed has said goodbye without giving first team football a chance.
Sir Alex makes very few errors, but in my opinion, this is a grave one. With young players like Zeki Fryers and Danny Welbeck still to make their minds up on a new contract, we shall be holding our breath until David Gill announces reassuring news.
What is most worrying is that Pogba clearly believes his prospects are better elsewhere. Otherwise he would at least have waited to see who Sir Alex signs before making up his mind.
Maybe he was reading all the Bleacher articles about talented midfielders due to be arriving at Old Trafford any time soon and decided that he had no chance of ever becoming the first-choice midfielder that Sir Alex hinted he could have become.
He's already had to warm the bench while men twice his age have been drafted in to salvage the season. If he can't get a game ahead of a 39-year-old in a lower-table Premier League match or a lesser European match, what did he think his prospects would be with younger men coming in?
Frankly, Sir Alex and United have got this one badly wrong. Samir Nasri may have chosen City for an extra £40,000 a week last summer, but Pogba has turned down an offer from United that matched Juventus and meant he could stay with his wave of mates likely to make the journey into the first-team squad.
No doubt the powers that be at Old Trafford will seek to blame the agent this time.
But if Paul Pogba really is one of the best players of his generation—his form massively confirmed that potential week in and week out, first in the Academy and then the Reserves—Sir Alex has got this one badly wrong.
There is nobody else at Old Trafford of a similar age with anything like the talent or potential. Will Keane, Zeki Fryers and Davide Petrucci will undoubtedly become established first-team players; Danny Welbeck could become the greatest English striker of his generation.
But in years to come, will we be moaning about "the one that got away"?
And here's the thing. This isn't a Ryan Shawcross or a Gerard Pique. This potentially Patrick Vieira, Roy Keane, Paul Scholes and Eden Hazard rolled into one.