"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."
Ronaldo wore the No 28 shirt at his boyhood club in Portugal but described how Ferguson sprung a surprise by asking him to pull on the No 7 - a number famously worn by David Beckham, Eric Cantona and George Best.
"I remember when I left Lisbon for England, Alex Ferguson said to me, 'Listen, I want you to take the No 7'," Ronaldo, speaking at the launch of his new Nike boots, added.
"I was surprised because I knew the guys who wore that shirt. I accepted the challenge, it was great.
"It was the beginning of my success in football. It was probably one of the most beautiful moments in my career so far."