ROME: Former World Player of the Year Ronaldo has admitted for the first time that retirement is a possibility if his knee fails to improve.

The 32 year-old underwent surgery on his knee in February 2008 and has been without a club since that injury negated his contract with AC Milan.

Ronaldo did receive a pay-as-you-score offer from Siena, but the former Barcelona, Inter Milan and Real Madrid striker turned it down as he was weighing up his future.

He told La Gazzetta dello Sport: "I''m thankful to him (Siena president Giovanni Lombardi Stronati), but I''m not sure what the future holds. I don''t know if I''ll continue to play or retire. I haven''t yet made that decision."

For the moment the Brazilian is concentrating on finishing his recovery, a process not helped by his fluctuating weight levels, but only once he has some idea of how well the recovery has gone will he make a decision.

"I don''t know, I''m not in a hurry. I want to finish my recovery work and then you will know. I could decide tomorrow or in a few months. I really don''t know.

"After an operation on a ruptured knee tendon you need to work for some months. The recovery is exhausting," he added.