The three-times World Player of the Year was carried off in tears after going down badly during Milan's 1-1 Serie A draw at home to Livorno on Wednesday. The 31-year-old, who has hardly featured this season because of a series of injuries, had only come on as a substitute three minutes earlier.

"AC Milan communicate that after the first checks made at the Galeazzi hospital in Milan a rupture to his kneecap ligament was found in his left knee," the club said in a statement on their website.

Gazetta dello Sport daily claimed Ronaldo’s career could be over as well.

“It’s an injury that could end the career of ‘Il Fenomeno’,” it said on its website. “The recovery time is dramatic: from nine months to a year.”

The Brazilian sustained two serious 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 what happened to Ronaldo," Milan coach Carlo Ancelotti said.

"I don't want to say his career is over because only time can put the word end to his career. From our part, we can only stay close to him. His team mates left the stadium quickly to reach the hospital to check on his condition.
‘Delicate moment’

"I think it's the right way to stay close to a player who's suffering. Being close to him can help him cope at such a delicate moment of his career," added the Milan coach.

Milan’s club doctor Jean-Pierre Messerman later confirmed Ronaldo had indeed suffered the same injury that inflicted his right knee eight years ago. Another doctor, Walter Pascale, however, did have some good news for Ronaldo.

"Surgical techniques to repair this kind of injury have improved a lot over the last eight years," he said. "If things go as they should he should get back to his best. However it's difficult at this time to say for how long he will be out."

Ronaldo fell awkwardly just three minutes after replacing Alberto Gilardino in the second half of the match. He was challenging for a ball in the air with Jose Vidigal, although the move resulted in a penalty to Milan which Andrea Pirlo converted to level the scores.

"I heard a horrible sound, like a bang, it was a strange sound," said Livorno goalkeeper Marco Amelia, who was close by when Ronaldo fell to the ground.

Ronaldo is the all-time leading scorer in World Cup finals matches with 15 goals in three tournaments. His contract at Milan runs out at the end of the season.