The news is a further blow to the Italy international, who has been riddled by injury issues over the last three years.
Rossi's bad luck started when he sustained a damaged anterior cruciate ligament to his right knee while playing for former club Villarreal, before suffering a recurrence in April 2012.
The 27-year-old scored 16 goals in 21 Serie A appearances last term, but suffered a sprain to his medial collateral ligament in January and, despite fighting hard to recover, was omitted from Italy's World Cup squad.
Rossi then complained of discomfort during pre-season prompting Fiorentina to send him for keyhole surgery in Colorado, and the 27-year-old faces another spell out in order to recover from the procedure.
"Giuseppe Rossi underwent keyhole surgery on his right knee in Vail, Colorado," read a statement on Fiorentina's website.
"The operation, performed by Dr S. Singleton, revealed good stability of the cruciate ligament that was previously operated on. During the surgery, it was also possible to suture repair his recent medial meniscus injury.
"Rossi will remain in Vail this week for further monitoring. After that he will begin his physiotherapy in New York before continuing in Florence.
"The striker's return to sports activity will be determined by the specialist and is estimated to be in four to five months' time."