Moyes is perceived as a man cut from the same cloth as Ferguson, who's shown loyalty, a faith in youth, and the ability to stick at a long-term project. He's punched well above his weight for a long time at Everton, and has long been heralded as a potential successor, and someone who could be in the job for many years.
In short, Moyes has won nothing. His Everton side have also never qualified for the Champions League (though they finished top four once), leaving him with no experience at the highest level of the game. Even his credentials over youth are suspect, with few of the young talents Everton have had during his tenure maturing into great players under his watch.
Verdict: The current favourite, although it would be a surprise were United to overlook the obvious shortcomings.
He's José Mourinho. The only man for whom the sheer scale of the role would not be intimidating, as well as a proven winner who has spoken of his admiration for both Ferguson and United in the past. He would all but guarantee a title on the table every year, can bring European success to the club, and can stamp his own personality on the team.
There are question marks over Mourinho's conduct (Bobby Charlton, now one of the United directors, is known not to be a fan), his ability to oversee a long-term project, and his role at working with young players. He may not represent much of a gamble on the purely on-pitch aspect of things, but there are many more questions surrounding that that could rule him out.
Verdict: Possible, although not as heavily-backed as Moyes, even if it does seem like a no-brainer.
Bringing through youth players: Check. Loyalty: Check. Overseeing a long-term project: Check. Winning trophies: Check. Managing a club at the highest level Check. Of all the candidates, Jurgen Klopp is the only one that ticks every box available, and Ferguson is clearly an admirer of his Borussia Dortmund side - as evidenced by the fact he keeps attempting to sign their players
Firstly, Klopp has never managed in England before. Secondly, although he has managed at the top level, he hasn't done so for very long and has relinquished the title to Bayern Munich this year almost without a fight. Lastly, it remains to be seen whether the time is right, or if it's a year or two too soon for the German.
Verdict: A dark horse, less likely than David Moyes or Jose Mourinho, but a contender nonetheless.
He's managed at top clubs all around the world and has brought a degree of success with him wherever he's been. He's also managed in England before, where he won the title with Chelsea, and oversaw a longer-term project at AC Milan, the club he's most assosciated with.
Although not quite on the brink of retirement, Ancelotti is not a young man, so his longevity will be under question. In addition, his reign at Milan only saw him win the league once, and with no connection to Manchester United, he has rarely been cited as one of the candidates for the succession.
Verdict: Highly unlikely. Keep your money on the other three contenders.