"The patient's status, from the cardiological point of view is good and he's slowly regaining health but it's not strong enough to let him leave today," said spokesman Andrzej Rusek.
The 70-year-old, Portugal's football ambassador, was taken to hospital in the Polish city on Saturday after feeling unwell. He has been watching matches at Euro 2012.
Eusebio has been admitted to hospital three times since December. He has been suffering from hypertension and his doctors have said he must have regular examinations.
Eusebio travelled with the national team to Poland and was in the stands for the quarter-final win over the Czech Republic, cheering alongside former Portugal captain Luis Figo. Portugal face Spain in the semi-finals on Wednesday.
Nicknamed the 'black panther', Eusebio grabbed international headlines during the 1966 World Cup in England when Portugal finished third. He also helped his Portuguese club side Benfica reach four European Cup finals in the 1960s.
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