Dinamo Zagreb midfielder Arijan Ademi has failed a drugs test which taken after he played against Arsenal in the Champions League.
Ademi played for 90 minutes during Zagreb's 2-1 victory over the Gunners and will face disciplinary proceedings if his B-sample is also positive.
"We are aware of the results of the doping control test after the match of the Champions League first round against Arsenal and that our player, Arijan Ademi, was positive, and that Uefa opened proceedings against him," a statement issued by Dinamo Zagreb said.
"According to Uefa rules, Dinamo cannot comment on the case until the B sample is given. Our players have been regularly subjected to doping controls for years after European and league games, and never has anything like this ever happened, so that we have to say that we are surprised by this result.
"Arijan Ademi was tested six times in the last year and each time the report was negative."
Zagreb will not face a points deduction if Ademi is deemed guilty.
Jonathan McIntosh
