Real Madrid midfielder Kaka risked his career by playing for Brazil at the World Cup, according to his doctor.
A routine medical check revealed the player would need arthroscopic surgery and Kaka flew straight to Antwerp on Wednesday night for the procedure.
The doctor who performed the surgery, Marc Martens, knew straight away the injury was serious.
"As soon as I saw him I could tell it was serious," the specialist told Marca.
"His career has come under serious danger."
Martens believes Kaka pushed himself too hard in South Africa.
"He started playing with some niggling injuries and ended up with unbearable pains," he said.
"I understand that it's a unique competition and that there was pressure, but he pushed himself much too hard against Holland and Chile. He admitted he was in a lot of pain."
Martens claims the injury could have spelt the end for Kaka as a professional footballer.
"It could have finished his career because it's a very serious injury - it could have destroyed him," he said.
"But thank God he has had the operation at the crucial moment and everything has turned out wonderfully," he said.
The news is less wonderful for Madrid, however, with the player set to miss three to four months as he recovers from the surgery.
A statement on the club's official website, www.realmadrid.com, said: "The operation performed on the left knee of Real Madrid CF's Ricardo Izecson Dos Santos Leite 'Kaka' by Dr Marc Martens at Antwerp's AZ Monica Hospital was a success.
"The arthroscopic surgery confirmed an injury to the left meniscus with minimal inflammation of the cartilage. The player is expected to miss three to four months."
The player himself also said the surgery had been a success and thanked the fans for their messages of support.
"I am grateful to everyone for their support - the arthroscopy has gone well," he said on Twitter.
Kaka joined Real Madrid from AC Milan in a £56million deal last summer, but suffered all season with injuries and started just 25 of the club's 38 Primera Division matches.
The 28-year-old was substituted in 14 of those and appeared a shadow of his former self in the time he did spend on the pitch.
The South American may find it difficult to win back his place when he returns, with the team set for some significant reshaping under new coach Jose Mourinho.
"If we have to lose him for a while, it won't be a drama - we have other players," Mourinho said after Madrid's 3-2 friendly win over Mexican side America on Wednesday night.
