Ferguson signed Juventus striker Ronaldo as a teenager from Sporting CP.
He told the Guardian: "Ronaldo is tough, honestly, like old boots. He would always be a great player, because he had everything up there.
"We played an important role in that, because Eric Harrison, the youth coach, made everything tougher for the youngsters. He said: 'If you are not mentally tough, you will never reach the United team'.
"People have an opinion of Manchester United in terms of great players like Ronaldo and Keane. But the spirit of that club, at my height, passed through young people, such as Beckham, Giggs, Scholes and the Nevilles."