The banner showed a masked man holding a blood-covered machete in his right hand, a la Friday The 13th villain Jason Voorhees, and the severed head of Defour in the other.
It was accompanied by the words "Red or Dead" (red is the color of Standard).
"This banner was in bad taste and unacceptable to Standard and Anderlecht," tweeted Bob Madou, the Belgian football federation's communications chief.
"There is nothing in the rules against it but we will see what we can do about it."
Thibault de Gendt, the director of the tournament, condemned the actions of Liege fans.
"It was disgusting and this took place in front of children. We hope never to see this again," he said.
Defour was playing for arch-rivals Anderlecht in the game which his new side lost 2-0 and ended with him literally seeing red after he was sent off early in the second half.
Defour, who skippered Standard to two Belgian league titles, joined Anderlecht earlier this season after three years with Portuguese club Porto, a switch that caused widespread anger in Liege.
