Sassuolo won the clash 2-1, but it was marred by Constant leaving the pitch over claims of racist abuse.
On 34 minutes, Constant suddenly kicked the ball into the stands and stormed off into the locker room. He said he heard a racist insult, but nobody else on the field appeared to. Milan sent on a substitute and the referee followed protocol by sending out a message over the tannoy system warning the game could be suspended.
It was similar to the incident that saw Kevin-Prince Boateng leave during a midweek friendly against Pro Patria, but that was after repeated racist chanting and the rest of the team followed him.