Blues sources had always insisted Costa was happy at Stamford Bridge and fight for his place under incoming boss Antonio Conte.
But Costa has now told the club he wants to go - and Atletico have already made their move for the Spain star.
It is understood a £40m initial bid for Costa is now on the table, a fee which could rise with add-ons, with both clubs set for talks over his future in the next few days.
Costa, 27, has also been open about the fact that he would relish a reunion with Diego Simeone at last season's Champions League runners-up.
Colombia international Martinez was moved on to China's Guanzhou Evergrande for £31m, just eight months after arriving at Atletico from Portuguese side Porto.
Although Chelsea are set to confirm the £33m capture of Marseille striker Michy Batshuayi on a five-year deal, they also retain a strong interest in Real Madrid’s £40m-rated marksman Alvaro Morata
