Dinamo Zagreb midfielder Arijan Ademi failed a drug test after their recent Champions League win over Arsenal, the club has announced.
Ademi, 24, played the full 90 minutes during the Croatian side's 2-1 victory in Zagreb last month.
Dinamo announced that Uefa has started disciplinary proceedings.
"We are surprised because Ademi was selected for doping control six times this year and results were negative every time," a club statement read.
If Ademi's B sample is also positive, the player could face a ban, but there are no rules to disqualify teams from the competition unless two or more players fail drug tests.
Arsenal have lost both games in Group F after a 3-2 defeat at home to Greek side Olympiakos, and still have to play German champions Bayern Munich home and away.
Macedonia international Ademi also played in Dinamo's 5-0 defeat by Bayern last week.