The central defender was given his marching orders in the closing stages of the clash in Rome for grabbing Stefano Mauri by the throat as tempers flared.
While Mauri was booked for his involvement in the fracas, Mexes saw red to cap a miserable night for Milan who lost 3-1, despite taking the lead after just four minutes.
The France international issued a full apology for his actions a day later, but the FIGC have come down hard on the defender with a four-game ban for violent conduct.
That will see the 32-year-old miss league encounters against relegation-threatened trio Parma, Empoli and Cesena, plus their trip to leaders Juventus.
His absence will be a big blow for the already under-pressure Filippo Inzaghi, who reportedly needs a victory in the Coppa Italia - against Lazio on Tuesday - to keep him at San Siro.
