It was perhaps the worst piece of career advice ever given out by a school teacher: "Forget the ball, the ball will not feed you."
Ronaldo's own revelation comes in a new book to be launched by the Reds star next week, entitled "Moments; The Way I Feel."
Not an autobiography, the book gives an insight into the Portuguese ace's life and inner thoughts - and it sheds light on a remarkable friendship which the multi-millionaire footballer struck up with a seven-year-old victim of the 2004 Indian Ocean tsunami.
An extract from the book, published on Ronaldo's agency website, tells of how he has changed little from his school days on the island of Madeira, when he got into trouble for being too fond of his football.
He says of the fifth grade teacher who scolded him for his attachment to his ball: "Today, I cannot stop myself from smiling while recalling her concern whenever I came to class, sometimes late, with the ball in one hand.
"`Ronaldo, forget the ball,' she would say over and over again. `The ball will not feed you. Do not miss classes. School is what really matters to you, not the ball, that will not bring you anything in life.'"
Ronaldo is now one of the Premiership's richest players after inspiring United to last season's Premiership title and lifting both main Footballer of the Year awards.
"Life is full of surprises," continues Ronaldo. "At the time I would listen to her without paying much attention. But today I understand her and, although she keeps on saying to my mother and my aunt that she will never make this type of comment again to a student, I still think she did the right thing and that she must keep on following her belief.
"As a teacher, she did her job, and it was a good piece of advice, as we never know what tomorrow brings. But I never paid much attention."