There is a worst-case scenario that must, even now, seven weeks before David Moyes takes over as Manchester United manager, haunt both him and those who appointed him. What if he starts badly?
What if, by the end of September, United have lost two or three league games and had a couple of iffy results in the Uefa Champions League?
It can happen - even the best sides can get off to slow starts - but if it happened for Moyes the consequences would be far more severe than if it happened for Sir Alex Ferguson, or even if it happened for Jose Mourinho, who seems to have been the other leading candidate to take charge.
Immediately, the whispers of doubt would begin to circulate. Is he up to it? Can he handle a big club? Is he tactically smart enough to cope in Europe?
Once the doubt is there, it can become self-perpetuating. As the great Hungarian coach Bela Guttmann once observed, being a coach is like being a lion tamer: the slightest sign of fear can be fatal.
Once players lose faith in a manager, once they begin to question orders they would previously have carried out instinctively, the manager is lost.
The truth is, nobody really has any idea whether Moyes is up to it because he has no experience at that level. His European experience amounts to 26 games, only two of them in the Champions League, both of which were lost.
That doubt is why so many of the Premier League's top teams, when looking for a new manager, turn to somebody who has already proved themselves in the Champions League - which in turn imposes a glass ceiling on the development of British coaches.
And that doubt is real enough: at the end of March, when it was put to one United director that Moyes might be the man to replace Ferguson, he rocked his head back and laughed at the prospect.
Yet if any British manager deserves a chance to prove he can cut it at the highest level, it is Moyes.
His 15-year managerial career has been one of sustained achievement on limited means.
He took over Preston North End in the third flight and led them into the Championship, and then to the play-offs for a spot in the Premier League, despite working with a far lower budget than many of the clubs he was competing against.
That earned him a move to Everton in 2002.
He lowered the average age of the squad, and improved the league position: only twice since his arrival have Everton finished outside the top ten, and this season they should finish above Liverpool for the second successive season for the first time in over half a century.
It is significant that when Moyes took the call to ask if he wanted to replace Walter Smith, he was in his car driving to Bristol to scout the young Bristol Rovers striker Nathan Ellington.
He has always had great energy, a determination to do things properly and, in part because of the resources he had to work with, a desire to focus on youth.
When David Gill, the United chief executive, was asked about the attributes he would be looking for in Ferguson's replacement, he listed loyalty and a willingness to develop young talent.
As the third-longest serving Premier League manager, and having brought through Wayne Rooney, Jack Rodwell, Seamus Coleman and Victor Anichebe, Moyes ticks both those boxes.
Like Ferguson, Moyes is a workaholic. He is a great analyst of statistics and videos.
Like Ferguson, he is a perfectionist, always looking for small advantages, always ready to stand his ground on the touchline.
Like Ferguson, he oversaw his club's move from a dilapidated and outdated training ground to a new custom-built one.
Like Ferguson, he is essentially a pragmatist. It is easy to see why he should be considered the man to replace him.
Moyes has been described as a conservative choice and, perhaps compared to Mourinho, who would have arrived with swagger and courted controversy in the role, he is.
But in going for somebody lower down the English structure, in not turning to an off-the-shelf manager with European experience, United have taken a gamble.
The truth is that, after Ferguson, there probably are no safe options.
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