A player from Serbian club FK Vojvodina has alleged he was attacked in the changing room by his own fans following a Europa League tie in Slovakia.
Midfielder Nikola Trujic, 25, said the assault happened after his side were eliminated in qualifying on 6 July.
The world players' union Fifpro has asked Uefa to open disciplinary proceedings against Vojvodina and their opponents Ruzomberok of Slovakia.
Vojvodina lost the second leg of the tie 2-0 to exit 3-2 on aggregate.
A Fifpro statement said that immediately after the game "a group of the Serbian club's supporters was able to gain access to the away team's (Vojvodina) changing room, before intimidating and physically assaulting the player".
Trujic also told Serbian media after the incident that about a dozen fans were involved. "They caught me by the neck, I got a blow to my head," he said.
Neither Uefa nor the two clubs have commented on the alleged assault.
Ruzomberok - who beat Norwegian side Brann in the next round after knocking out Vojvodina - face Everton in the third round of Europa League qualifying.