Aberdeen defender Jack MacKenzie required stitches after being struck by a seat thrown after the match against Dundee United.
The defender needed medical attention after a chair was lobbed onto the pitch and struck him after the match at Tannadice.
It's thought the chair was thrown from the Aberdeen section at Tannadice. Images show MacKenzie clutching his face after falling to the ground when the object hit him.
Dundee United supporters had ran onto the pitch after sealing European football before the incident took place.
MacKenzie could be seen being treated for his injuries after the shocking incident with his head bandaged.
A seat was then removed from the pitch by a security guard at Tannadice.
"I just spoke with him, he's okay but it's quite a bad injury," Jimmy Thelin told BBC Scotland.
"It's unacceptable it happened to a football player who is just playing football and gives so much for us.
"This is unacceptable and can't happen.
"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, performing for Aberdeen football club.
"This cannot happen in this way."
Dundee United boss Jim Goodwin said of the incident: "I deliberately took myself off the pitch as quickly as I could because we've had previous with the Aberdeen supporters in the past - I've been hit with a number of objects in the fixtures we've played.
"I'm really disappointed to hear young Jack MacKenzie 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 the idiots who let down a really good club."
An SPFL spokesperson said: “It’s enormously disappointing and frustrating that one of the most exciting games in the season has been marred by such moronic behaviour.
“We await the match delegate report but will work closely with both clubs and Police Scotland to help identify the perpetrator and fully expect that appropriately severe footballing and judicial sanctions will be imposed.”
An Aberdeen statement read: "The Club is dismayed and outraged at the scenes that unfolded following this afternoon’s Premiership tie with Dundee United at Tannadice Park.
"Our player, Jack MacKenzie, has sustained a serious facial injury after being struck by a part of a chair that was thrown towards the pitch.
"We await further information, but what we know for sure, the chair was thrown from the stand in which the Aberdeen supporters were being housed.
"As such, we are calling on all fans who may have witnessed this event to contact the Club to provide any information that may assist us in our investigations and wider discussions with the relevant authorities.
"When the perpetrator is identified, we will take the strongest possible action available to us."
