MILAN, Italy -- AC Milan forward Ronaldo ruptured a tendon in his left knee in a 1-1 draw with Livorno on Wednesday, and the Brazilian was carried off the field in tears.
The injury occurred two minutes after Ronaldo entered the game during a play that earned Milan an equalizing penalty.
Ronaldo was taken to a hospital, with the club saying the full extent of the damage was unknown.
"It's a problem with the tendon in his left knee, not the knee that was operated on in the past," Milan vice president Adriano Galliani told Italy's Sky TV. "The severity of it isn't known yet, but it's a serious problem."
A three-time FIFA world player of the year, Ronaldo has battled injuries throughout his career.
He has played sparingly since joining Milan from Real Madrid in January 2007. Wednesday's match was only his fifth in Serie A this season.
Ronaldo injured his thigh July 31 during preseason training. He returned in November but has been on and off the roster due to a series of recurring problems.