Anderlecht chairman Roger Vanden Stock has condemned the behaviour of Standard Liege's supporters after a banner depicting Steven Defour's severed head.
The midfielder was making his first appearance since leaving Standard in 2011, having since returned to Belgium with fierce rivals Anderlecht after a spell with Porto.
A banner showing a figure holding a sword in one hand and Defour's head in the other was unfurled prior to kick off, adorned with the words "Red or Dead".
Following further taunts during the match, Defour reacted by kicking the ball towards a section of Standard's fans, earning him a second yellow card.
Defour later apologised for his actions, but Vanden Stock has laid the blame firmly on their Pro League rivals.
In an open letter on the club's official website he said: "What happened on Sunday afternoon in Liege has left us - the management, the staff and the players of Royal Sporting Club Anderlecht - numb.
"I have never experienced something like this, as a football fan, and as president of the most successful club in the country.
"The image of the Belgian club football has gotten a nasty bite, an important border has been crossed.
"I also understand that you feel the fans, cheated. The outrage is deep.
"It is not the first time that we have to play football in abnormal conditions in Liege. Probably the incidents of yesterday cost us three points, maybe not. We will never know.
"Despite that frustration, I would ask you to remain worthy to stay in Anderlecht. Your attitude carries longstanding to the greatness of our club.
"Bad taste has no place in the Belgian stadiums and certainly not in the Constant Vanden Stock stadium."
With a man extra Standard went on to win the game 2-0, leaving Anderlecht five points off leaders Club Brugge.