His Milan counterpart Clarence Seedorf added: "I hope they find the culprit and do what they have to do.
"I can only compliment my players who continued to play and got on with their jobs."
Constant, 26, walked off the pitch in response to alleged racist chanting during a pre-season friendly against fellow Serie A side Sassuolo in July.
And in January 2013, Kevin Prince-Boateng – now with Schalke – threw the ball into the crowd and walked off the pitch after fans of lower-league Pro Patria hurled racist abuse at him during a friendly match in Busto Arsizio.
The match was subsequently abandoned and six Pro Patria fans were convicted of racial abuse and imprisoned.