Galatasaray have accused FC Copenhagen goalkeeper Kamil Grabara of acting against Uefa's fair play principles and expect him to be punished by his club.
Grabara, 24, posted a derogatory comment about Galatasaray on social media after his side were held to a 2-2 draw in Turkey on Wednesday.
The Pole received death threats and his wife shared screenshots of threatening messages she had been sent.
Galatasaray said they attributed his words "to his youth and inexperience".
"The statements [Grabara] made before and after the match are against Uefa's fair play principles," the Turkish club said.
"We expect the Copenhagen club, which we hosted for two days, to give him the necessary punishment."
Grabara, who was previously at Liverpool from 2017-2021 but did not play for the first team, told reporters before the game he had "heard Fenerbahce fans are better" than their Galatasaray counterparts.
Following the match, in which Copenhagen threw away a two-goal lead in the final four minutes of normal time, he used an expletive to refer to Galatasaray - before later editing the post to use a similar expletive to describe the game.
Alongside evidence of the death threats directed at Grabara and his family, his wife said on Instagram: "This is not normal. 300 messages and I can't count how many comments. There are no words."
BBC Sport has approached FC Copenhagen for comment.