Centre-back Daniele Bonera was on Monday ruled out for up to three months after breaking his kneecap during Saturday's friendly victory over Valencia, and full-back De Sciglio and France international Mexes have now been ruled out of the forthcoming Guinness International Champions Cup tournament in the United States.
The pair will also miss the chance to face Bayern Munich, Manchester City and Sao Paulo in the Audi Cup, which is being hosted by the European champions this week.
A statement published on Milan's official website on Tuesday read: "AC Milan announce that Mattia De Sciglio and Philippe Mexes have not been called up for the Audi Cup and Guinness International Champions Cup due to respective physical problems.
"De Sciglio has twisted his left knee and will undergo further tests in 10 to 15 days. Mexes has suffered a recurrence of a muscle problem."
Brazilian striker Robinho, who on Monday picked up an adductor problem, will also be unavailable in Munich and the International Champions Cup, where the likes of Chelsea, Real Madrid, Juventus, and Los Angeles Galaxy will do battle in venues across the USA.