Piatti was taken to hospital with a double fracture of his right arm and he is now expected to be out for at least two months.
The 23-year-old Argentinian will definitely be unavailable for the opening games against Bayern Munich and Lille, as well as the double header against Bate Borisov.
He is also doubtful for the home clash with Bayern on November 20.
"Piatti is going to remain in hospital for the next 48 hours and, save any complications, will be discharged after this period," read a club statement.
"Due to the precaution needed in these cases, the estimated recovery period is between eight and 10 weeks."
Piatti made 31 La Liga appearances last season, scoring three goals.