AC Milan and Brazil striker Ronaldo ruptured a ligament in his left knee and must undergo surgery, the Serie A club said.
The three-times World Player of the Year was carried off in tears after going down heavily during the 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 on their Web site (www.acmilan.com).
Ronaldo 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.
VERY SAD
"We're all very sad and worried about what happened to 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.
"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.
"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 president Silvio Berlusconi said the Brazilian is flying to France on Thursday for surgery.
"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.
