Sports daily Gazzetta dello Sport reported that the 31-year-old Ronaldo may be out of action 9-12 months and the injury might end his career.
Ronaldo, whose recent career has been beset by injuries, had been only on the field for three minutes as a substitute when he went down to the ground after jumping in the air for a ball in the Livorno penalty area.
"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."
Ronaldo suffered two serious injuries to his right knee that kept him out of action for 18 months in 1999-2000, but he returned to lead Brazil to the 2002 World Cup title.
After four-plus seasons at Real Madrid, Ronaldo was sold to AC Milan 13 months ago. He scored seven goals in 14 games for Milan last season, but injuries limited him to only 320 minutes of play in six league games this season.
"When Ronie cried, I hugged him, the tears fell on me too, but I told him we would stay close to him," Adriano Galliani, Milan's managing director. "Milan have already shown in the past they don't abandon players who suffer injuries, and the same will go for Ronaldo.''
1993-94 Cruzeiro (Brazil) 14 games, 12 goals
1994-96 PSV (Netherlands) 45 games, 42 goals
1996-97 Barcelona (Spain) 37 games, 34 goals
1997-2002 Inter Milan (Italy) 68 games, 49 goals
2002-07 Real Madrid (Spain) 127 games, 85 goals
2007- AC Milan (Italy) 20 games, 9 goals
(Statistics in league play.)