Sturm Graz issued an apology after the assistant referee sustained a cut to the head in their Europa League tie against AEK Larnaca and said the person responsible has been caught.
Fredrik Klyver appeared to be struck by a cup thrown from the crowd during Thursday's tie at Merkur Arena, leading to a long delay as he received treatment.
The teams were taken off the field by referee Mohammed Al-Hakim and, when they emerged, Klyver had been replaced by the fourth official.
"This behaviour of a fan in today's Europa League game is by no means acceptable," said a Sturm Graz statement released after the match.