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 tonight.
"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.
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 goal scorer in FIFA 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, just three minutes after replacing Alberto Gilardino in the second half.
Milan head of medicine Jean Pierre Meersseman confirmed the extent of the player's injury.
He said: "After Ronaldo's injury we decided at once to transfer the player to the Galeazzi hospital in Milan. It is one of the best orthopaedic hospitals in Europe.
"There's little to say about the seriousness of the injury - unfortunately they confirmed that it was a rupture to the tendons of his left knee. He will definitely need surgery.
"We are all distraught at what's happened."
Livorno goalkeeper Marco Amelia, who was close by when Ronaldo fell to the ground, said: "I heard a horrible sound, like a bang, it was a strange sound."