He was treated for bronchial pneumonia in December and has been admitted to hospital on several occasions since.
He has been suffering from hypertension and his doctors have said he must have regular examinations.
The former striker played for nine clubs during a 22-year professional career but is most remembered for his 15 years with Benfica and a goal-laden international career.
Born in Mozambique in 1942 when it was still a Portuguese colony, Eusebio da Silva Ferreira went on to play 64 times for Portugal, scoring 41 goals and winning the Golden Boot as the 1966 World Cup's top goalscorer.
He won 10 league championships, five Portuguese cups and one European Cup in his 15 years at Benfica and was Portugal's top league scorer between 1964 and 1973.
Eusebio scores four goals for Portugal against North Korea in 1966
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