Costa is still in his native Brazil, having been told he has no future at Stamford Bridge. He has been widely linked with a move back to Atletico, with whom he won La Liga in 2014 before joining Chelsea.
Atletico's pursuit is muddied by the fact they cannot field new signings until January 1, 2018, due to a FIFA transfer embargo, meaning Costa will have to play the first half of the season elsewhere before switching to the capital.
Nevertheless, Cerezo remains unbowed and is still hopeful a deal can be struck.
He told Cadena Ser: "He has a contract with another club, it’s a complicated matter.
"We are taking things calmly. But as long as Chelsea do not agree to his transfer, Costa will be a Chelsea player."
Asked if he was pessimistic or optimistic on agreeing a deal, he added: "I am always optimistic."
Costa has been photographed looking short of fitness in recent weeks but Cerezo does not think that's a problem.
He said: "On holiday it is normal to get a little fat, but I understand Costa is a professional."