Ronaldo has ruptured a tendon in his right knee during AC Milan's 1-1 draw at home to Livornoand faces nine months on the sidelines, according to Gazetta dello Sport.
AC Milan confirmed the extent of Ronaldo's injury on their website, which he suffered just minutes after entering the fray as a second-half substitute and was taken to hospital.
Gazetta claimed the former three-time world player of the year'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."
Milan coach Carlo Ancelotti refused to call time on Ronaldo's career, though.
"We're all very sorry and worried about what happened to Ronaldo, I don't want to say it's the end of his career because only time can decide whether it is or not," he said.
"From our part all we can do is stand by him and let him recover. Our players all left the stadium in a hurry to go to the hospital to check on his condition, I think that's the right way to stand by a player who's suffering."
Ronaldo was taken to hospital from where Milan's Brazilian scout Leonardo, said the striker had all but confirmed those worst fears.
Ronaldo underwent an X-ray at hospital and Milan's club doctor Jean-Pierre Messerman later confirmed he 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 tenchniques 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 will go under the knife in Paris today where he also underwent surgery in 1999. He fell awkwardly just three minutes after replacing Alberto Gilardino in the second half yesterday.
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.
The last time Ronaldo suffered the same injury he managed to make his comeback in April 2000 but lasted only seven minutes before reinjuring his knee.
It was 20 months before he made a proper comeback for the 2002 World Cup, during which he finished as top scorer as he guided Brazil to a record fifth triumph. However Ronaldo is now 31 and his contract at Milan runs out at the end of the season.