Balotelli ruled out for around 10 days
The Italy international was substituted 12 minutes from time in Milan's 1-0 UEFA Champions League defeat against Atletico Madrid on Wednesday after being fouled by Diego Costa and was sent for tests on Thursday.
The Serie A club have confirmed that the former Manchester City front-man will miss the trip to Sampdoria on Sunday and faces a race against time to be fit to face leaders Juventus seven days later.
Mattia De Sciglio suffered an ankle problem in the clash against Atletico and it is not yet known how long the defender will be sidelined for.
A statement released by Milan on Thursday said: "AC Milan wishes to communicate that in yesterday evening's match, Mario Balotelli suffered a shoulder injury.
"The player visited with professor Alessandro Castagna today and recovery time has been estimated at around 10 days without unforeseen complications.
"Mattia De Sciglio also suffered an ankle injury in yesterday’s match and the player's recovery time will be decided on in the coming days based on medical tests."