A Burnley fan was stabbed, and four others were injured, before the first leg of their Europa League play-off against Olympiakos in Greece.
The fan needed stitches to a leg wound and received medical attention at the stadium near Athens.
Around 900 Burnley fans travelled to Greece for the game; which ended in a 3-1 loss.
A statement from the club continued "However, five of our supporters were unfortunately injured - including a person who received a stab wound to the leg - and received medical attention at the stadium."
"Those injured all travelled independently to the stadium, outside the organised travel operation."
The club said they were working closely with Olympiakos and local police in Greece to investigate the incidents.