Sir Alex Ferguson has claimed only four of his Manchester United players could be truly regarded as 'world-class'. Ferguson confirmed Cristiano Ronaldo as the greatest player he worked with at Old Trafford. He also put Eric Cantona, Ryan Giggs and Paul Scholes among the elite, as he reflected on his career in his new book 'Leading'. Ferguson said: 'If you read the papers or listen to the television commentators, we seem to be awash with 'world-class' footballers. 'In my book there are only two world-class players playing today: Lionel Messi and Cristiano Ronaldo. '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 signed Ronaldo as a teenager for £12.25million from Sporting Lisbon in 2003 and reluctantly sold him to Real Madrid for £80m six years later. There was no place in the very top bracket for England stars like Wayne Rooney, David Beckham or Rio Ferdinand. Ferguson spoke at the Royal Festival Hall on Monday night on the first of four theatre dates to accompany the release of his book, co-written with successful businessman Sir Michael Moritz, and he paid tribute to another tier of his United players. 'There were big influences like Bryan Robson, Roy Keane and Steve Bruce,' said Ferguson. 'They were not the type of have egos. They weren't the best footballers but they had wonderful desire and made sure they were the best players.' Read more: Sir Alex Ferguson worked with just 'four world class players'