MILAN, Feb 14 (Reuters) - A knee operation undertaken on AC Milan and Brazil striker Ronaldo went smoothly on Thursday, according to his medical specialist in Paris. The three-times World Player of the Year, who ruptured a ligament in his left knee on Wednesday, was treated at the Pitie-Salpetriere hospital in the French capital. Knee specialist Gerard Saillant, who oversaw the operation and performed surgery on Ronaldo's right knee eight years ago, told La Gazzetta dello Sport: 'The operation lasted around two hours and took place without any particular problems. 'It was the same injury as the previous time and for this reason the operation was of the same type. 'Now Ronaldo will have to go through nine months of rehabilitation.' Asked if the Brazilian would play again, Saillant replied: 'It will depend on him.' The 31-year-old was carried off in tears after going down heavily during Wednesday's 1-1 Serie A draw at home to Livorno. Ronaldo, who has hardly featured this season because of a series of injuries, had only come on as a substitute three minutes earlier. Ronaldo sustained two bad injuries to his right knee while playing for Inter Milan in 1999 and 2000 but recovered to help Brazil win the 2002 World Cup. He went on to play for Real Madrid before joining AC Milan in January 2007. 'We're all very sad and worried about Ronaldo,' said Milan coach Carlo Ancelotti. 'I don't want to say his career is over because only time can put the word end to his career. 'His team mates left the stadium quickly to reach the hospital to check on his condition. Being close to him can help him cope at such a delicate moment of his career.' Milan president Silvio Berlusconi said he believed Ronaldo would be able to resume his career. 'He still has an extraordinary physique. I'm confident in some months time he can return to be the player we all know,' Berlusconi told Rai television. Ronaldo is the all-time leading scorer in World Cup finals with 15 goals in three tournaments.