Ronaldo's departure is far from the end of the world for Manchester United. Yes, the Barclays Premier League champions have lost one of the world's best footballers, and yes, if Sir Alex Ferguson could have kept hold of Ronaldo's precocious talent, he almost certainly would have done. But the fact remains the Portugal captain's transfer to the Bernabeu completes a remarkable piece of business for United, writes Claire Harmer.

He arrived at Old Trafford in 2003, a wonder kid certainly, but far from the finished article. Even at 18 years old he was commanding world record fees; his £12.24m transfer from Sporting Lisbon was the most ever paid for a teenager.

Now, having completed his apprenticeship at the Premier League's most successful club, that sum looks meagre in comparison to his current valuation. In June 2009, United accepted a world record bid of £80m for the winger, a whopping sum equal to the combined total of the club's six previous top sales: David Beckham, £25m to Real Madrid, Jaap Stam, £16.5m Lazio, Juan Sebastian Veron, £15m Chelsea, Andy Cole, £8m Blackburn Rovers, and Paul Ince, £7m Liverpool.

But even after making a phenomenal profit of £68m, Sir Alex Ferguson will have a tough time replacing the midfield ace.

Ronaldo seemed destined for great things the moment he arrived at the Theatre of Dreams and was handed the number 7 shirt - worn by past greats George Best, Bryan Robson, Eric Cantona and David Beckham. He vowed to live up to shirt's history, saying: "The famous shirt was an extra source of motivation. I was forced to live up to such an honour," and of course, he did just that.

Ever the showman, Ronaldo was king of the step-over in his first Premier League season. Criticism from some sections for his over elaborate footwork was like water off a duck's back for the youngster, who made 29 appearances in his first season, scoring four goals in the process. With Ferguson and his coaching staff on hand to rein him in on occasion, Ronaldo learned to use his tricks sparingly, only when they would have maximum impact.

Even in those early days Ronaldo proved he was no one-trick pony. He had the double bluff, backheeling the ball from left to right, tricking his opponent as he changed direction at lightning speed. His pin-point perfect free-kicks left goalkeepers dazed and confused as he netted stunning strike after stunning strike.

He could pass the ball precisely, even when looking or pointing in the opposite direction, another clever guise used to perplex his opponents. Even without the showboating, his sheer pace proved formidable, dazzling supporters and opponents alike with his unstoppable force.

And then there were the goals. It took him 10 top-flight matches to first find the back of the net for United, a 30-yard free kick at Fratton Park that dipped in mid air, proving impossible to defend against. After that stunning effort, Ronaldo continued to wow the Premier League in his role as match-winner, free-kick specialist and even magician.

He started slowly, netting 18 top-flight goals in his first three seasons, a respectable return for a winger of his age. And then, as the 2006/07 season began, Ronaldo suddenly found goals easier to come by, scoring 17 league goals that year. He won the Barclays Golden Boot award the following season, surpassing every out and out striker in the league with an outstanding tally of 31 goals from 34 matches. In total, he scored 117 goals from 289 appearances for the Red Devils.

He proved he could turn a match, an invaluable asset whenever title hunting United looked like faltering from their usual high standards. Take his performance against Arsenal in February 2005. With United trailing 2-1 at the break, he scored two in five minutes to propel his side in to the lead, with the match finally ending 4-2 to the Red Devils. Or, two years later, with his side struggling to get ahead against Fulham, a superb individual effort late on finally sealed the win, sending United nine points clear at the Premier League summit.

Then there was his superb effort against Portsmouth in January 2008. Having scored the opener, he struck again with a trade-mark dipping and bending free-kick, which his manager later described as one of the finest he had ever seen. And just a few weeks previously, he claimed his first hat-trick for United in the 6-0 annihilation of Newcastle United.

All of this without even mentioning his exploits in the Champions League, where he really seemed to raise the bar, saving his best for the biggest occasions. His goals against Roma, Chelsea and in particular, Porto, deserve a mention simply for their brilliance.

But with all of that talent, came an ego that rival supporters found tough to warm to. Safe in the knowledge that he was one of the best on display in any match he played, Ronaldo's self confidence personified itself on the pitch, often through his over-exuberant goal celebrations, his tantrums, or his attempts at gamesmanship and simulation to gain his team an unfair advantage. As a result, he often became the object of opposing fans' disdain, frequently taking on the role of a pantomime villain.

Ronaldo himself believes the crowd's taunts and jeers made him a better player. He said: "I love it when people jeer me. I love to see the hate in their eyes, to hear the insults.

"It's true lots of people hate me but there are even more who love me. I feel bad only when I play badly. Fortunately, that happens rarely."

After his Madrid move was confirmed, he revealed with amazing self belief, his ultimate objective.

"I really want to re-write the history of football," he said.

"I'm already among the best of the best but I want to continue to write many more beautiful pages."

"I am Cristiano Ronaldo - and I can win more medals than anybody else. I don't like to relax. The fans want to see a great Cristiano Ronaldo so I try to never disappoint."

In six seasons with United, Ronaldo proved he certainly could win more trophies than anybody else. He won the Premier League three times, the Champions League, FA Cup and FIFA Club World Cup once, not to mention two League Cups and the FA Community shield. Among his many personal accolades are the Ballon d'Or, with which he became Manchester United's first beneficiary since George Best in 1968, the 2007/08 Barclays Player of the Year, the European Golden Shoe 2007/08, and the UEFA Club Footballer of the Year 2007/08. And in 2008 he became the first Premier League player ever to be named FIFA World Player of the Year.

There may only be one Cristiano Ronaldo, but you would not want to bet against the arrival of a player of equal or perhaps even greater talent making his way to Old Trafford sooner rather than later. An exciting summer transfer window is now wide open, and we wait with baited breath to see who Sir Alex Ferguson will bring in.