Fiziev threw a kick with his right leg in the second round of the fight with Mateusz Gamrot and went down in pain immediately afterward. On the kick, Fiziev planted his left knee and it gave way.
Gamrot was awarded a TKO victory via injury stoppage.
Typically, fighters are able to return to the cage within nine to 12 months after tearing an ACL.
ESPN had Fiziev (12-3) ranked No. 7 in the world at lightweight entering the bout.
He was on a six-fight winning streak prior to a loss to Justin Gaethje at UFC 286 in March. The 30-year-old Kazakhstan-born fighter, who trains out of Florida, had won six straight $50,000 fight-night bonuses prior to the Gamrot bout.