The Real Madrid forward move to Old Trafford aged 18 from boyhood club Sporting Lisbon and ended up winning the FA Cup, three Premier League titles and the Champions League.
Since then he has had continued success in Madrid, adding two further Champions League successes, a Liga title and his fourth Ballon d’Or crown.
But, during the launch of some new boots, the 32-year-old's thoughts returned to his time in England and how much he picked up in the Premier League.
"Alex Ferguson was a father for me in football," Ronaldo said. "I learned many things, not just from the coach but the players.
"I improved not just my skills but my body too.
In the five years that I was [at United] I learned the art of football. It was a dream for me.
Read the full article on eurosport.co.uk: Ronaldo: I learnt art of football at United, Fergie was a father figure