Dinamo Zagreb midfielder Arijan Ademi failed a doping test after the Croatian side's Champions League win over Arsenal last month, his club have announced.
The 24-year-old played the full 90 minutes of Dinamo's 2-1 home victory in Group F.
His club said in a statement on their website: "UEFA started disciplinary proceedings against our player Arijan Ademi. Due to a positive doping test after the Champions league match Dinamo - Arsenal, UEFA instigates disciplinary proceedings against Ademi.
"Our players are constantly under doping control, especially during the UEFA competitions, and this is the first time we are facing a situation like this one.
"We are surprised because Ademi was selected for doping control six times this year and results were negative every time, as well as results of every other player who was selected for doping control by UEFA or Croatian Anti-Doping Agency.
"During a disciplinary procedure GNK Dinamo nor the player are not allowed to make any statement. We will inform you about the progress of this situation."
The match result is not expected to be affected by the outcome of tests on Ademi's B sample.
UEFA has yet to comment on the case.
Macedonia international Ademi also played in Dinamo's 5-0 defeat at the hands of Bayern Munich last week.