The 41-year-old, who also played for clubs including Barcelona, Inter and AC Milan, was reportedly taken ill on holiday on the White Isle
Ronaldo is in intensive care in Ibiza after being struck down with pneumonia.
The former Barcelona, Real Madrid and Brazil striker, 41, confirmed he had been admitted to hospital, reportedly Can Misses, on the Spanish island on Friday night.
He asked to be transferred to a private hospital, Clinica Nuestra Señora del Rosario, just before midnight on the same day, reports local newspaper Diario de Ibiza.
On Sunday afternoon, Ronaldo said he expected to be discharged on Monday.
His tweet translates as: “Friends, I had a strong flu picture in Ibiza and I had to be boarded on Friday but it’s all in order.
“I get the discharge tomorrow and come back home. Thank you all for your love and your messages!”
Ronaldo, regarded as one of the world’s best ever players, owns a house in Ibiza and is a regular visitor to the island.
He is believed to have arrived on the White Isle for a summer break a few days ago.
Former Manchester United and England centre-half Rio Ferdinand wished him well after hearing the news.
Ronaldo went down with the tropical disease dengue fever in January 2012, later posting a picture of himself on Twitter hooked up to a drip in hospital with the message: “Good morning to all those who began the New Year with dengue.”
He retired from football in 2011 after a glittering career, which also included spells at Cruzeiro, PSV Eindhoven, AC Milan and Corinthians.
Since hanging up his boots, a two-time Ballon d’Or winner – whose full name is Ronaldo Luis Nazario de Lima – has struggled with his weight due to a reported thyroid problem.
Pneumonia is swelling of the tissue in one or both lungs, usually caused by a bacterial infection.
Symptoms, which include a cough and difficulty breathing, can develop suddenly over 24 to 48 hours.
It can be life-threatening, especially among the elderly and those with underlying health conditions.