The 23-year-old fell into hot water with ultra group Curva Nord when he criticised one of its members in his autobiography, citing an incident when he tried to give a young fan his shirt after a defeat to Sassuolo in the 2014-15 season.
The club captain also claimed that he could call on some criminals from Argentina to kill the ultras, which has sparked anger among the supporters.
"The situation between Icardi and the Curva definitely had an effect," De Boer told reporters. "It is not good for the team, the fans or the club.
"Everyone needs to make sure similar situations can be avoided in the future, not just Icardi. The staff and all the players need to stay focused on the match in hand - that is very important.
"Mauro needs to take on board what happened and remember his responsibilities as captain. I think he understands that. He needs to set a good example for his teammates."
Reports have claimed that Paris Saint-Germain will offer Icardi a lifeline by making a bid for the striker in the January transfer window.