Sir Alex Ferguson won everything there was to win during his 27-year spell as Manchester United manager.

The legendary Scotsman won an incredible 38 trophies with the Red Devils, including 13 Premier League trophies and two Champions League titles.

Of course, while Ferguson was an incredible manager, he wouldn't have been able to do it without some of the best players in the world.

Ferguson managed so many incredible players during his time at the club.

However, he once expressed his belief that he only managed four world class players.

"In my book there are only two world-class players playing today: Lionel Messi and Cristiano Ronaldo,” Ferguson wrote in his 2015 book 'Leading', per the Independent.

"I don't mean to demean or criticise any of the great or very good footballers who played for me during my 26-year career at United, but there were only four who were world class: Cantona, Giggs, Ronaldo and Scholes.

"And of the four Cristiano was like an ornament on the top of a Christmas tree."

Ferguson later said in an interview with the BBC: "They made the difference and the evidence is there. When we brought him [Cantona] in we won the league that season - it was his mere presence and his ability to make and score goals."

"The younger breed like Ryan and Scholes were just fantastic players and the thing about those two was longevity.

"Are there players who have played right through the whole of the Premier League and performed at the level they have? There are none, absolutely none.

"Of course Ronaldo was just a complete genius of a player."

Those comments were controversial to say the least.

Roy Keane is arguably the greatest captain in Premier League history and he guided the club to so much success.

Rio Ferdinand and Nemanja Vidic were both brilliant and formed one of the best centre-back partnerships in Premier League history.

Wayne Rooney is one of United's greatest ever players, while David Beckham, Peter Schmeichel and Patrice Evra could all have arguments to be described as 'world class'.

Nevertheless, Ferguson is arguably the greatest manager of all time so it's almost impossible to argue against his opinion.