Antonio Cassano suffered a cerebral problem related to an ischaemic stroke, his club AC Milan said on Wednesday.
He is due to undergo an operation to close a small hole in his heart and is expected to be out for several months.
"Having consulted with the Policlinico (hospital) in Milan, AC Milan can confirm that Antonio Cassano suffered from a cerebral problem related to an ischaemic stroke," said the Milan statement.
"The instrumental and neurological examinations needed 72 hours to be developed and demonstrated a small problem in the cerebral area which has not caused lasting damage.
"The cause has been identified as a hole between the (left and right) atriums (in the heart) which could only be detected by sophisticated equipment.
"Therapy should result in a rapid recovery and improvement in his clinical conidition, which is good.
"The footballer will undergo in the coming days a small heart operation (to close the hole) and the recovery period before returning to sporting activity will be several months."
Cassano's condition is not a particularly rare one which usually causes little to no affects in the sufferer.
It is essentially an anomaly of the heart in which a small hole develops between the left and right atriums allowing blood to escape between the two, causing turbulence that can result in blood clotting.
Cassano was taken ill on Milan's return from Rome in the early hours of Sunday morning, following a 3-2 win over Roma, and the 29-year-old was admitted to the Policlino hospital in Milan.
He was suffering from difficulties in his speech and movement.
In hospital Cassano was transferred to the neurological department where he was to undergo tests on his heart and brain.
Milan chief executive Adriano Galliani moved to quell fears this could mean an end to the 29-year-old's playing career.
"Antonio's in good condition, he's well. I'm not a doctor but the statement says everything," said Galliani.
"This problem will be resolved through surgery and then in several months it will be the Italian Football Federation who give him the green light to start playing again.
"I was very, very worried but there's no risk to his career. The doctors say he needs several months, not so many to recover but so that he can follow the process.
"I don't want to make any predictions but surely between four to six months he should have his licence to play.
"Antonio's joking as per usual, he's missing football, he told me he's disappointed because he was doing well and I told him to stay calm because his position (with Milan) is secure."