The Sun says the Brazilian has started training again and hopes to shed the excess weight, as he eyes a move to mega-rich City.
Ronaldo, 32, said: "When you haven't been able to do any physical activity for five months, it's only natural to put on weight.
"I'm not feeling any twinges in my knee anymore and that's what really matters.
"The muscles are still weak though and it's not easy to get a machine going that hasn't been used for so long.
"From now on it's about working on my physical condition - that's the trickiest part but I'm on the right track.
"I will be back and I'll rediscover my best form. I'll play football again like I did before."