Joao Havelange was described as being in a "serious condition" Tuesday by doctors treating the former FIFA president for a bacterial infection at a Rio hospital.
"The condition of the honorary president of FIFA is serious and must be treated with caution," doctor Joao Mansur Filho was quoted as saying in a statement released by the Samaritano hospital.
Echoing a statement released on Monday, it added that tests revealed "a serious bacterial infection, with the presence of cellulitis in the right ankle".
Havelange, who served as FIFA president from 1974 to 1998, is held in high esteem by some but is regarded as a controversial figure by others.
He is credited with turning football into the moneymaking industry it is today but resigned as a member of the International Olympic Committee after 48 years just days before an ethics hearing into allegations of corruption.
Havelange is also the former father-in-law of Ricardo Teixeira, who recently resigned as head of Brazilian football and as chief of Brazil's organizing committee for the 2014 World Cup following a spate of corruption allegations.
He was under investigation for his links with FIFA's former marketing agency, International Sport and Leisure (ISL), which went bankrupt in 2001 with debts of around $300 million.
Havelange competed at two Olympic Games in 1936 in Berlin as a swimmer and then in the 1952 edition in Helsinki as a member of Brazil's water polo team.
He was also instrumental in bringing the 2016 Olympics to Rio de Janeiro and to South America for the first time when the IOC elected the city as the host in 2009 in Copenhagen.