Cairo says Okaka is too good to spend time on Toro's bench.
He explained: "Okaka is an important player, and should be bought as a starter," he told an Italian radio station.
"As we already have Belotti, I don't think he's the man for us."
Meanwhile, Watford defender Brandon Mason has signed a new three-year contract.
Mason, who joined Scottish Championship side Dundee United on loan until the end of the season last week, confirmed to the Dundee Evening Telegraph that there are still more than two years left to run on his deal at Vicarage Road.
"My situation is being reviewed at the end of the season but I still have two years left on my contract so we will have discussions when I go back in the summer. Getting first-team experience here can definitely help me."