The 31-year-old underwent a two-hour operation in Paris on Thursday after suffering ruptured knee ligaments winning the penalty which earned his side a point in Wednesday's 1-1 Serie A draw with Livorno.

Gerard Saillant, the same doctor who treated Ronaldo when he suffered a similar injury in 2000, confirmed the operation had gone well but could not absolve fears that the Brazil striker's career could be over.

"It went well," said Saillant. "The operation lasted for around two hours and finished without any extra problems.

"Now Ronaldo has to undergo a rehabilitation programme which will last nine months.

"Considering his age and the fact that he is eight years older than the last injury, it is difficult to say when he can return to playing.

"Will he be able to continue playing? That will depend on him.

"It's evident that his return to the pitch will depend on how the rehabilitation goes but also on his desire.

"The psychological factor is almost as important as the physical one.

"In short it depends on him."

Ronaldo was accompanied to Paris by his fiancee, club official and compatriot Leonardo, club doctor Massimiliano Sala and physiotherapist Bruno Mazziotti.

The striker, who was operated on by Eric Rolland and his assistant Yves Catonne with Saillant acting as a consultant, is expected to remain in hospital for at least 10 days.

Saillant confirmed that Ronaldo will definitely be out for nine to 12 months.

Italian media have speculated that the 2002 World Cup-winner could have kicked his last ball professionally after the latest in a catalogue of serious knee injuries.

But Piero Volpi, Internazionale's club doctor from 1995 to 2000, believes otherwise.

"It's a very serious injury," he told SkyTg 24. "But the tear can repair itself.

"Today even better than before because surgical techniques have improved.

"Recuperation time is eight to 10 months but I am not pessimistic about the future of Ronaldo's career.

"The real problem is that he is eight years older than in 2000 and for an athlete it is always more difficult to return to full strength."

With his contract set to end on June 30, Ronaldo could have played his last game for AC Milan.

But AC Milan president Silvio Berlusconi earlier backed the Brazilian to return better than ever from his latest injury heartbreak.

"I reached Ronaldo by phone yesterday and I told him to believe in himself," he told television programme Unomattina.

"He has an extraordinary physique and at 31 years is still young.

"I believe that in some months he can return as the champion that we know he is and that is the wish of everyone in the sporting world."