At the age of eight, Ronaldo played for amateur team Andorinha, where his father was the kit man. In 1995, Ronaldo signed with local club Nacional, and, after a title-winning campaign, he went on a three-day trial with Sporting CP, who subsequently signed him for an undisclosed sum.
Ronaldo joined Sporting's other youth players who trained at the Academia Sporting, the club's football academy, in Alcochete. He became the only player ever to play for Sporting's U-16, U-17, U-18, B-team, and first team, all within one season. He scored two goals in his Sporting debut against Moreirense, while featuring for Portugal in the 2002 European Under-17 Championship.
When he was 15, Ronaldo was diagnosed with a racing heart, a condition that might have forced him to give up playing football. The Sporting staff were made aware of the condition and Ronaldo's mother gave her authorisation for him to go into hospital. While there, he had an operation in which a laser was used to cauterise the area of his heart that was causing the problem. The surgery took place in the morning and Ronaldo was discharged from hospital by the end of the afternoon; he resumed training only a few days later.
He was first spotted by then-Liverpool manager Gérard Houllier at the age of 16, but Liverpool declined to take him on because they decided he was too
young and needed some time to develop his skills. However, he came to the attention of Manchester United manager Alex Ferguson in the summer of 2003, when Sporting defeated United 3–1 in the inauguration of the Estádio José Alvalade in Lisbon. Ronaldo's performance impressed the Manchester United players, who urged Ferguson to sign him.
Ronaldo became Manchester United's first-ever Portuguese player when he signed for £12.24 million after the 2002–03 season. He requested the number 28 (his number at Sporting), as he did not want the pressure of living up to the expectation linked to the number 7 shirt, which had previously been worn by players such as George Best, Bryan Robson, Eric Cantona, and David Beckham. "After I joined, the manager asked me