A Burnley supporter was stabbed in the leg ahead of the team's Europa League clash away to Olympiacos.
Burnley confirmed in a club statement that the fan received medical attention at the Georgios Karaiskakis Stadium on Thursday.
The Premier League side said four other visiting supporters were also injured and treated at the ground.
"Those injured all travelled independently to the stadium, outside the organised travel operation," Burnley's statement read.
"Burnley are working closely with Olympiacos and the local Greek police to investigate these incidents."
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Burnley also confirmed 22 fans were held by police for drunken behaviour, although they were later released.
"A group of 22 supporters were detained by police ahead of the game for offences relating to drunkenness, in breach of Greek legislation," the club said.
"They were detained, rather than arrested, and released later on Thursday night without charge."
Burnley lost 3-1, with captain Konstantinos Fortounis scoring twice to give the Greek side a strong lead ahead of next week's return leg at Turf Moor.
The winners of the tie will progress to the group stage of the Europa League.
