The Holland striker is expected to leave for a bigger club this summer.
Sartori said, "Did I say before that Zirkzee will leave 99% of the time? I have known the market for many years and I believe there is a strong possibility that he could leave, he is a great player and the attacker is a highly sought-after role.
"We've been thinking about it for months because replacing him will be very difficult but we'll try. We have a few names and we'll work to replace him."
Sartori also discussed the arrivals of wing-backs Juan Miranda (Real Betis) and Emil Holm (Spezia).
He added: "We needed to intervene on the wings and we did it in a very short time. Now we stop and evaluate what the market offers. The priority could be the replacement of Zirkzee, we must be ready."