Balotelli picked up the muscle problem in his right calf during Wednesday's 2-0 win over the Los Angeles Galaxy, with tests required when he returns to Italy with the Milan squad today.
Milan president Adriano Galliani said: "During the course of the game, Mario Balotelli suffered a muscular problem in his right thigh.
"He will be examined on his return to Italy. Balotelli will have some exams and hopefully it is nothing too serious.
"Let us be patient because no one wants to take a player out of international duty."
The 22-year-old is now considered doubtful for Italy's friendly against Argentina next week, with one eye on Milan's Champions League qualifier against PSV Eindhoven later this month.