"AC Milan announces that Alessio Romagnoli today underwent a medical examination that revealed an injury to his right thigh flexor," a Milan statement confirmed.
The news is a major blow to head coach Vincenzo Montella, who has been forced to field a makeshift defence in Milan's last two matches.
Romagnoli was used at left-back for the 1-0 defeat to Sampdoria at San Siro on February 5, with Cristian Zapata and Gabriel Paletta partnered in the middle and Juraj Kucka deployed as an emergency right-back.
Paletta was sent off during the 1-0 win over Bologna this week, leaving Montella desperately short of options now that Romagnoli is expected to be sidelined.