But Pele's personal assistant Jose Fornos moved quickly to play down the news by explaining he had simply been moved to a private room.
And the three-time World Cup winner himself later released a statement on Twitter to assure his followers he was on the mend.
He wrote: "Hello everyone, I want to take this opportunity to let you know that I am doing fine.
"I was not put into intensive care today, I was simply relocated to a special room within the hospital for privacy purposes only.
"I am blessed to receive your love and support, and thank God this is nothing serious.
"I am looking forward to spending the upcoming holidays with my family, and will start the new year with renewed health, with many international trips already planned! Thank you!"
A statement from the hospital earlier, reported by Globoesporte.com, read: "The Hospital Israelita Albert Einstein reports that the patient Edson Arantes do Nascimento [Pele] has been admitted with clinical instability.
"To receive the best care, he has been transferred to be monitored in a special care unit. The hospital will update when there is new information."
"It will take between eight and 10 days before he can leave the hospital," he added to Globoesporte.com.
Pele underwent surgery at the same hospital two weeks ago to have kidney stones removed.
The issue forced the cancellation of an appearance at the Pele Museum in Santos due to "health reasons".
He has previously been under the care of the Albert Einstein hospital having had thigh surgery two years ago.
Pele won the World Cup with Brazil in 1958, 1962 and 1970 over a 14-year international career
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