Costa, has been away to his home town of Lagarto in Brazil.
He had told the Daily Mail UK on Sunday that Chelsea are treating him “like a criminal” and that he has lost all personal respect for head coach Antonio Conte.
Costa had also disclosed back in June that Conte, via text message had told him he would not be in Chelsea’s plans for the new season.
The Spain international was granted an extra few days off when the rest of the Chelsea squad gathered at Cobham for preseason training in early July.
Costa has refused to come back to training since after the expiration of his holiday and claimed on Sunday that the club has asked him to return and train away from his teammates.
According to ESPN, no such condition was laid out to Costa.
The club simply expect the 28-year-old to fulfill the terms of his contract by coming back to training, regaining match fitness and making himself available for selection.
Chelsea have fined the striker around £300,000 which is his two weeks’ wages for his refusal to return to England.