The Portuguese forward impressed at Manchester United with his desire to improve himself as a player and that has continued throughout his career
Cristiano Ronaldo's work ethic developed at Manchester United continues to be rewarded with Real Madrid
During the 2003-04 season, Manchester United’s assistant manager Carlos Queiroz called Cristiano Ronaldo into his office at the club’s Carrington training ground.
“Look, this is very simple,” the coach told him. “It's not enough for you to be a great player. You need to understand that God gave you the skills and opportunity to be the best player in the world.
"If you want to work in that direction you have my full support. If you want to be just another player, I'll treat you like I treat the others.”
Queiroz knew how Ronaldo would respond.
“It was very rare to see in a young player so much talent and such a strong personality, purpose and commitment. I read about Arnold Schwarzenegger learning to dance the tango. He was obsessed to be a perfectionist. Cristiano is the same.”
That dedication would be spelled out in action rather than words, as Queiroz often witnessed.
“I was once in my office at Carrington and saw something moving in the trees far away. Maybe it was a spy. I called security and asked him. He came back to me and said: ‘It’s Cristiano Ronaldo. He’s training alone’. He was unique.”
Fifteen years on, Ronaldo, 33, is continuing to drive himself on to bigger and better things in football.
His two goals against Juventus on Tuesday in Real Madrid's 3-0 triumph over Juventus were not only sublime, but they were his 42nd and 43rd goals in the 33 Champions League games since the club last lost a tie in the competition in 2015.
Only Barcelona's Lionel Messi comes close. Ronaldo was so good against Juventus, the last team to beat Madrid, that the home fans applauded his second goal, a spectacular bicycle kick.
Juventus fans know quality when they see it. They also applauded Manchester United after they came from 2-0 down against their side to win 3-2 in the semi-final of the 1999 Champions League.
United would have loved to have kept Ronaldo, who departed in 2009, but the Portuguese was keen to play for a team who have won the European Cup far more times than anyone else.
Madrid fans know quality, too. They applauded Barcelona’s Ronaldinho after he played a two-goal starring role in a 2005 Bernabeu clasico.
The Bernabeu also applauded Barcelona’s Diego Maradona in the 1980s and Andres Iniesta, a hero of Spain, when he was substituted as his side led 4-0 in 2015.
It is easier to be magnanimous when your team is winning, less so when you are 4-0 down at home to your greatest rivals.
Madrid fans may even bring themselves to applaud Messi one day - the same could be said of Camp Nou and Ronaldo.
Another Ronaldo, the Brazilian striker, was applauded by United fans after scoring a hat-trick at Old Trafford for Real Madrid.
“I’ll never forget that night at Old Trafford, the only people who didn’t clap Ronaldo were Alex (Ferguson) and I,” said Queiroz.
“Even our directors applauded. That was a tribute to the United fans, the English fans. It was a tribute to football. And of course to Ronaldo.”
Tuesday’s emphatic victory over Juventus means Zinedine Zidane’s side are clear favourites to reach an eighth successive semi-final and keep them on track to be the first side since Bayern in 1976 to win the European Cup three times in a row.
Such consistency is helped when they have a player of Ronaldo’s talents and commitment in key matches to come up with the big goals.
How different it could have been for United if they had manage to keep him.
How different it would have been for them if they had not signed him in that summer of 2003.