MILAN, Italy – Ronaldo is still unsure whether he will return to professional football, and his 22-minute appearance in a charity match this week hasn't helped him decide.

The three-time FIFA player of the year ruptured a tendon in his left knee in a Serie A game with AC Milan in February – the third major knee injury of his career – and is still in rehab.

After a brief appearance in the Match Against Poverty, a charity game sponsored by the United Nations on Monday in Fez, Morocco, Ronaldo said he felt “very tired.”

“I only played 22 minutes because I had a muscle problem recently,” the Brazilian striker said in Wednesday's Gazzetta dello Sport. “I'm tired because the rehab work I've done is tough and my legs are heavy. Football is my life, but 22 minutes at that pace is not much more than an exhibition. Real professionalism is completely different.”

Ronaldo recently turned down an offer from Siena president Giovanni Stronati to play in Serie A again.

“I thank him, but I really don't know what the future holds for me,” Ronaldo said. “I don't know whether to continue playing or retire. I haven't taken a decision yet.

“There's no rush. I want to finish my rehab, and then you know how these things go. I could decide tomorrow or a month from now. I really don't know.”

The Gazzetta speculated that if he retires, Ronaldo could be offered a position by FIFA as a missionary ambassador to developing countries.

Ronaldo is already a goodwill ambassador for the UN Development Program.

“I consider myself a lucky man, and to be able to give back in some way seems the least I can do,” he said.