Ronaldo has tested positive for COVID-19 and was forced to miss Sunday's anniversary celebrations of Cruzeiro, the Brazilian club he recently bought.
The two-time Ballon d'Or winner, who top-scored in Brazil's 2002 World Cup triumph, has been experiencing mild symptoms of the coronavirus.
Ronaldo had been due to attend events marking Cruzeiro's 101st anniversary, but the 45-year-old former Barcelona, Real Madrid and Inter striker was instead isolating.
His positive case was announced by the Brazilian club, who were acquired by Ronaldo, with the support of an investment bank, in December.
"Cruzeiro reports that this Sunday morning Ronaldo Nazario tested positive for COVID-19," the Campeonato Serie B club said.
"This makes it impossible for him to travel to Belo Horizonte today, as well as his presence in the commemorative actions for the club's anniversary.
"Ronaldo is doing well, with mild symptoms and, on medical advice, is now in rest and social isolation."
Ronaldo has been majority owner of Spanish club Real Valladolid since 2018 and serves as the Segunda Division team's president. He had been due to meet with representatives of Cruzeiro supporters on Sunday and was scheduled to appear at a news conference on Monday before taking ill.
He began his playing career at Cruzeiro as a teenage sensation before heading to Europe to join Dutch giants PSV after the 1994 World Cup.
Ronaldo wrote on Twitter: "Cruzeiro nation, this is not the way I wanted to spend this January 2 which is more than special for us.
"I had the greatest anticipation and everything was organised for this moment. But there are situations in life that are beyond our control and, unfortunately, last night I found myself in one of them. I started to feel bad, had a PCR and woke up today with a positive result for COVID-19.
"The most important thing is that I am vaccinated, I will follow the isolation protocols and, at the end of it, I hope to be 100 per cent recovered to resume my schedule.
"Our meeting was not cancelled, it was postponed due to circumstances. And I make a point of celebrating with you the anniversary of our club! It's just the beginning of our fight together! I count on you, count on me."