Ronaldo's weight problems in recent years were caused by hypothyroidism, but pills and a new training regime have made the Brazil striker much fitter, his club AC Milan has said.
Ronaldo was ridiculed by fans and the press during his time at Real Madrid when his weight ballooned and he became known as "el gordo" (the fat one).
He moved to AC Milan in January in a bid to rebuild his career and has since lost 12 pounds.
The weight loss program, coupled with a hamstring injury, means he will not be fit for Sunday's Serie A kickoff at Genoa but he is expected to be back in three weeks.
"Soon you will see him, very soon. It will be a surprise for everyone. He seems like a model," Milan coach Carlo Ancelotti told Italian media on Friday.
"He will have a great season, he is very motivated and really wants to demonstrate again that he is the strongest attacker in the world."
Hypothyroidism is caused by insufficient production of hormones by the thyroid gland and can lead to weight gain.