Kaka played through the World Cup carrying the injury which has sidelined him for up to four months, the Real Madrid star revealed.
The Brazil international has been ruled out of a significant chunk of the new Primera Liga season after undergoing surgery on his left knee in Belgium on Thursday.
Kaka's doctor labelled the injury "career threatening" and voiced his concern that the 28-year-old was pushing himself too hard.
But the Madrid playmaker insists he never realised the severity of the problem.
He told the Spanish press: "This knee has been bothering me for quite some time. But I didn't know that it was so serious. I was the first to be surprised.
"I was always being treated for a groin strain and I was told that it could cause muscular imbalance in other parts, but I didn't know it was so serious."
Kaka flew straight to Antwerp on Wednesday night for the procedure, which was performed by surgeon Marc Martens.
"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 and stated that the injury could have ended his glittering career prematurely.
"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."
"It could have finished his career because it's a very serious injury - it could have destroyed him.
"But thank God he has had the operation at the crucial moment and everything
has turned out wonderfully."