The latest news comes with Stevan Jovetic struggling with a leg injury and veteran forward Luca Toni recovering from concussion, and Vincenzo Montella's men are starting to look worryingly light up front.
Serbia international midfielder Ljajic is not a natural striker but has acquitted himself well alongside Toni during the prolonged absence of Jovetic.
A statement released on the club's official website read: "Fiorentina can confirm that Adem Ljajic has suffered a lesion to his thigh during training.
"The prognosis is a break from training of around 15 to 20 days."
Mounir El Hamdaoui is now Fiorentina's only fit senior striker, meaning 20-year-old Haris Seferovic may be drafted in after scoring in Wednesday's 2-0 Coppa Italia victory over Juve Stabia.