The Germany international collided with Michael Agazzi during the Serie A clash and was substituted in the 49th minute.
In a statement, Fiorentina confirmed the 28-year-old had suffered a tear in his medial collateral ligament.
"Mario Gomez suffered a knee sprain with (a) partial tear of the medial collateral ligament," the club said on its website, while also confirming "further investigations" will be carried out over the next few days.
Fiorentina have yet to put a timescale on the forward's return to action.
Gomez moved to the Stadio Artemio Franchi from Bundesliga champions Bayern Munich in July and has scored twice in Serie A.
Juan Cuadrado was also hurt during the match, suffering a shoulder injury.
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