The hardest thing about getting to the summit of world football is staying there.

Once you've conquered them all, you're the one to beat and everyone is willing for you to trip up and regress back to mortality.

For Manchester United, that time could be on its way.

After dominating the Premier League for 20 years, they have found themselves struggling without their talismanic leader Sir Alex Ferguson, who retired from his Old Trafford post in the summer.

New manager David Moyes now faces a fight to emulate his compatriot and has so far failed to convince fans that he has that ability.

Although, of course, too early to tell if Manchester United are entering a period of decline, it has happened before.

Here we take a look at five massive downfalls that have occurred in the Premier League.

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