Doctors in Spain are concerned Atletico Madrid striker Diego Costa's latest medical problem could be related to his coronavirus diagnosis earlier this year.
Costa is suffering from deep vein thrombosis, a condition that occurs when a blood clot forms in one or more deep veins in the body, usually in the legs or pelvis.
The clot can loosen and travel through the bloodstream causing serious complications if it reaches the heart.
There is uncertainty as to whether Costa's coronavirus positive, suffered at the start of September, is related to the current problem. Concerned medics at the Madrid club have not put a date on the player's return.
Deep vein thrombosis has been noted in some Covid-19 patients. Doctors believe the immune system's excessive response to deal with the virus can generate clotting substances that increases the risk of thrombosis.
Doctor Jose Roman Escudero told Spanish radio: 'It has been shown that 30 percent of patients who have died from Covid have done so from thrombosis and pulmonary embolisms. Thrombosis and Covid are closely related.'
Asked how long Costa could be out for Escudero said: 'Being a footballer there is more contact and you have to be more prudent. At a minimum, the recovery time would be between 3 and 6 months in normal patients.'
Doctor Joaquim Gea Guiral told Spanish paper ABC: 'It could be associated with Covid, although thrombosis usually occurs in more serious conditions.'
Costa tested positive for coronavirus in September although at the time of his diagnosis he reported no symptoms.
Costa had expected to play against Lokomotiv Moscow on Wednesday but was a late withdrawal.
The club issued a statement shortly after confirming the deep vein thrombosis diagnosis and added: 'It is not related to trauma or previous injury and is pending evolution.'