Aberdeen defender Jack MacKenzie was struck by a seat seemingly thrown by the club's own fans after Saturday's defeat to Dundee United that led to a pitch invasion.
Dundee earned a 2-1 win to deny Aberdeen fourth place in the Scottish Premiership. MacKenzie was an unused substitute, but he was on the pitch after the game when a projectile -- thought to be a ripped out seat from the visiting end -- struck him.
Abderdeen boss Jimmy Thelin described the behaviour as "unacceptable," although he confirmed MacKenzie was doing okay.
"This is unacceptable and can't happen," Thelin told BBC Scotland.
"We have to get all the facts of what actually happened. All we know is Jack got a really bad injury as a player doing his job. This cannot happen in this way."
Dundee United boss Jim Goodwin, who was targeted with projectiles when the two sides last met at Tannadice, said the incident is a "disgrace."
"I deliberately took myself off the pitch as quickly as I could because we've had previous with the Aberdeen supporters in the past," Goodwin told BBC Scotland.
"I'm really disappointed to hear young Jack has been hit by an object thrown by his own supporters. He's inside having stitches now, it's an absolute disgrace.
"It's probably taken something as serious as this to happen for the authorities to finally step in and do something about it to identify these idiots who let down a really good club."