Esperance of Tunisia defender Houssem Dagdoug tested positive for an unspecified banned substance after an African Champions League match last month, his club said on Friday.
Esperance said on their website that the Confederation of African Football (CAF) had informed them of Dagdoug's failed test and that they were awaiting further details in the case.
The club added that CAF had not yet informed them of the consequences for Dagdoug.
Dagdoug, 24, was tested after Esperance played Al-Merrikh of Sudan in the first round of the group stage on February 11, a match in which the left back was an unused substitute.
He then played in two other group games.
Esperance are top of Group D on nine points, three ahead of Algeria's CR Belouizdad. Egyptian side Zamalek are in third place, level on four points with Al-Merrikh in fourth.