After defeating Atletico Madrid at the Estadio Wanda Metropolitano in midweek, AC Milan have lost for the second game in a row in Serie A.
This defeat could see them lose their place at the top of the table if Napoli can see off Lazio at home on Sunday night.
Things started well for the Rossoneri as they took the lead through Alessio Romagnoli, and they were playing well.
The lead was short-lived, however, as Gianluca Scamacca equalised with a fantastic long-range effort for Sassuolo, and things went from bad to worse as Simon Kjaer put the ball in his own net less than 10 minutes later.
The victory was wrapped up for the away side just after the hour mark with a good goal from Domenico Berardi on his 250th Serie A match. This was his seventh goal against Milan, who are now the side he's scored most against in his career.
Romagnoli was then given a straight red card with just over 10 minutes remaining in what was a miserable afternoon for Stefano Pioli's side.