Dinamo Zagreb midfielder Arijan Ademi has been banned for four years for failing a drugs test following the Champions League win over Arsenal.
The 24-year-old Macedonia international played the full 90 minutes of the shock victory in Zagreb on 16 September.
Ademi said: "I am not guilty and I didn't do anything wrong. I don't know what to say about this suspension. I feel awful right now."
Coach Zoran Mamic said the club will appeal and called it a "humiliation".
Mamic told the Zagreb website: "Ademi and experts proved his supplement was infected with a prohibited substance and he didn't know what kind of supplement he was using.
"That is why I can't understand this decision made by Uefa.
"A four-year suspension is a stupidity. How would Uefa punish Arsenal, Bayern, Manchester or some other big club?
"We will fight for him and for his case because Ademi is a brilliant young guy, player and a vice-captain.
"We want to protect him, protect the club and Croatian football in general, because this is a huge stain for all of us."
There are no rules to disqualify teams from the Champions League unless at least two players fail drugs tests.