The 31-year-old ruptured tendons in his left knee against Livorno and knows he must face up to the fact he may never play competitive football again.

"I have to be prepared for the possibility of retiring," Ronaldo told Brazilian television show Fantastico.

"I know it won't be easy. It will be a very tough recovery and something might happen during the recovery period which means I cannot play again.

"But I will leave that possibility aside until the end of the rehabilitation. If then my knee still hurts every day, even after it has mended, maybe it would be time to say goodbye to football.

"I have to make a good recovery so I can ride a bike with my son and run with him without pain."

The World Cup winner sustained a similar injury in his other knee in 1999 and was out for more than a year.

"As soon as I did the injury I wondered why it was happening to me again, what had I done to deserve it," he added.

Brazilian giants Flamengo have expressed their desire to sign Ronaldo as soon as he comes back from the injury, but the striker is reluctant to discuss the possibility because of the uncertainty currently shrouding his future.

"It is not the best moment to talk about Flamengo or Milan or any other club," he said.

"Logically, no club wants a 31-year-old player who has had two operations, suffered terrible injuries to his knees and is doubtful to play again.

"However, at the same time, I don't need any charity by any club. I just thank God for my history in football."