Ajax Amsterdam's Davy Klaassen suffered a head injury from an object thrown from the stands, causing a lengthy delay, before his team edged bitter rivals Feyenoord away in Rotterdam on Wednesday to book a place in the Dutch Cup final.
A man was arrested in the stands as Klaassen bled heavily from the wound and he was later substituted.
The match was halted for 28 minutes before being restarted and Ajax won 2-1 to set up a final against PSV Eindhoven on April 30.
Ajax were leading 2-1 when a confrontation between rival players near the corner flag set off a commotion, during which Klaassen was hit on the head, seemingly by a cigarette lighter.
Ajax players walked off the field, followed by those of Feyenoord, whose assistant coach John de Wolf later came out to berate home supporters before the game got back underway. He used the public address system to ask Feyenoord fans to "use your common sense".
Dutch international Klaassen, 30, had to be substituted shortly after the game resumed, gesturing that he was suffering from dizziness.
Police said a 32-year-man had been arrested and the incident was being investigated.
The cup semi-final, weeks after Feyenoord beat Ajax away in Amsterdam in a key league game to move close to the Dutch title, had started five minutes late due to smoke from fireworks lit by Feyenoord supporters. It had to be stopped 21 seconds after kick-off to allow the heavy black smoke to clear.