"Eusebio spent the night well but there was no significant improvement," said Luz hospital clinical director Jose Roquette. "We hope that all is solved quickly though this might take a while. It is likely that he will stay until after Christmas."
Eusebio, who turns 70 in January and is a national icon in Portugal, was admitted to hospital on Wednesday.
"Eusebio has been with his family and some friends," Roquette told reporters.
The Mozambique-born forward helped Benfica reach four European Cup finals, the last in 1968 when they lost in extra-time to Manchester United. Benfica won the trophy in 1961 and 1962.
He retired in 1975, wrapping up a career that included 64 caps and 41 international goals and an emotional Wembley appearance against England in the semi-finals of the 1966 World Cup, where he was top scorer.
Eusebio was visited by former Portugal national coach Humberto Coelho on Thursday.
"He was a bit sleepy but I managed to talk to him. Let's wait, but I hope he recovers fast. He is strong so I believe that will happen," said Coelho.
"It is hard to see him like this. On top of everything this is happening on Christmas, a more delicate moment. I am hoping he will recover... let's pray for him."
