Brazilian striker Ronaldo may retire from football following his latest serious knee injury, the 32-year-old said yesterday.

Ronaldo ruptured a tendon in his left knee playing for Milan in February and made his return earlier this week when he appeared for just 22 minutes in a charity match in Morocco.

His Milan contract ran out in the close season and offers from big clubs have failed to materialise for the Brazilian, who has turned down an approach from Serie A mid-table side Siena.

"I thank them but I really don't know what the future holds for me," Ronaldo told Gazzetta dello Sport.

"I don't know if I will continue playing or retire. I still haven't taken a decision. There is no rush. I want to complete my recuperation. I could decide tomorrow or in a month. I really don't know."

England's Manchester City have been linked with a move for the 1997 and 2002 Ballon d'Or winner, along with Brazil's Flamengo.

However, the former Real Madrid player is still nowhere near ready for action after struggling and looking overweight in Morocco.

"I am tired because my rehabilitation work has been hard and my legs are heavy," added Ronaldo, who suffered serious injuries to his right knee while playing for Inter in 1999 and 2000 but recovered to help Brazil win the 2002 World Cup.