AC Milan striker Ronaldo faces up to nine months on the sidelines after rupturing a tendon in his left knee.
The 31-year-old was carried off in tears after suffering the injury against Livorno on Wednesday.
"I don't want to say it's the end of his career because only time can decide - all we can do is stand by him," said Milan coach Carlo Ancelotti.
Milan face Premier League leaders Arsenal in the first leg of their last 16 Champions League tie on Wednesday.
Ronaldo, who is out of contract at the end of the season, suffered the same injury in his right knee in November 1999, lasted seven minutes on his comeback in April 2000 before taking another 20 months to make a full recovery.
Ancelotti said the whole club was worried about the Brazilian, who was named Fifa world player of the year three times and is the record scorer in World Cup finals.
"We're all very sorry and worried about what happened to Ronaldo," said Ancelotti.
"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 fell awkwardly while challenging for a ball in the air with Jose Vidigal, shortly after replacing Alberto Gilardino in the second half.
"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.
(BBC)
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