Sir Alex Ferguson is arguably the greatest manager of all time, and a tough act to follow.
But has he left David Moyes a 'poisoned chalice'?
After Sir Matt Busby it took seven attempts and 17 years to find a man to emulate his great achievements.
Despite being in his 70s it was still a shock to most people when Sir Alex retired. Many United supporters have only ever known him as manager. So it is no real surprise that he still seems virtually infallible.
For example, by choosing David Moyes and no doubt advising the United board to give the new man plenty of time to get things right. This may also substantially explain why both the media and most supporters are still giving Moyes the benefit of the doubt.
Since Sunday's defeat at the hands of Swansea there has been a growing swell of supporters questioning whether the ex-Evertonian is the right man for the job.
There are no doubt many theories as to why United are sliding so fast from their runaway title win last season. It is the same squad, so is it the manager or the players falling short? The only addition was the questionable £27million signing of Marouane Fellaini.
The harsh reality, however, is that the blame must surely be apportioned between Moyes and Sir Alex.
United may have won the title last year, but many supporters and commentators felt it was with one of their worst teams in a long while.
And yes it could be said that Moyes failed to make any meaningful additions, but in some respects that judgement would be harsh.
When Sir Alex announced his retirement almost four months before the start of this season, whose decision was it for his successor to start on 1 July? That left Moyes with less than seven weeks to get to know his new charges.
Many of them were on holiday or international duty, meaning that barely half his first team squad was on tour to the Far East. Understandably Moyes needed time to assess every player.
Was the need for a major rebuilding job the biggest influence on Sir Alex's decision to retire? After all, in 2001 when he previously decided to retire he knew he needed a new team. How many times can you find the energy to do that and still be at the pinnacle of success?
There is one aspect of the case for rebuilding that can firmly be laid at the door of the great Scot. That is the crying need for a new midfield. It wasn't just the ageing Paul Scholes that needed replacing; there have been cracks ever since Roy Keane left.
You cannot of course replace like with like; the game has moved on. But United undoubtedly need at least one holding and a creative midfielder. Then there is the matter of replacing the other squad stalwarts; players like Rio Ferdinand, Nemanja Vidic, Patrice Evra and Michael Carrick.
In fact when you get down to it there is a major rebuilding job to be done, but was Moyes the right man to do it?
Who in world football would have turned Sir Alex down when he told his fellow Scot he would succeed him? Of course the Glazers had to agree but the 'absent' US owners were hardly going to query the judgement of United's best manager of all time, particularly not after the conveyor belt of success he had delivered.
But here again, was Moyes the right choice? Surely Jurgen Klopp or an experienced man like Carlo Ancelotti or Louis van Gaal with an 'apprentice' number two such as Ryan Giggs or Gary Neville would have been a better choice.
Assuming Moyes didn't just get it because of his nationality, a key factor would be his previous record in developing young players at Everton. That ability was the mark of Sir Matt and Sir Alex. United are as good as anyone at bringing young talent through- just look at the 'Class of 92'.
As well as the midfield dearth and the ageing squad there is also the 'dead wood' to consider. It is indeed a testament to Sir Alex that he honed such a mediocre squad with the addition of Robin van Persie into yet another title winning side.
But would Jose Mourinho have been able to make more of the likes of Anderson, Ashley Young and Nani than Moyes has?
And it's not just the quality of some players, it is also the money that has been wasted. Sir Alex used the excuse of there being "no value in the market" but the truth is that he wasn't prepared to shell out for the 'marquee signings' that a globally worshipped team should demand.
Which means that someway or other Moyes must spend far more than the restrained Ferguson was prepared to in finding probably four or five top players to complement what developing talent there is within the squad and the Academy.
So while the knives are starting to emerge for Moyes, there are also rumblings of discontent at the hand that he was dealt by his predecessor.
Even before Rooney and Van Persie are fit again the manager will have to prove his competence in the transfer market, as well as his coaching and man-management credentials.
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