Romagnoli was replaced already in the fifth minute of the game with Sassuolo on Sunday (1: 1).
"FC Milan reports that Alessio Romagnoli was injured in the biceps muscle of his left thigh during a match against Sassuolo," says the statement of the "devils".
"The player will undergo further medical examinations within two weeks".
Romagnoli should replace Matteo Musacchio, who went out and to meet with Sassuolo, but Gennaro Gattuso needs to come up with who will replace another centripe, Leonardo Bonucci, who will not play against Napoli because of the disqualification.
In this season, Romagnoli played 25 matches in Serie A, which marked 2 goals.
Milan after the 31 round is the sixth, behind the zone of the Champions League by 8 points.