Demirci made the surprise move in January, becoming the first Dutchman to join the LaLiga leaders since Johnny Heitinga.
The youngster has been registered initially with the U19 team and told Voetbalzone: “Phew, yes. I still can hardly believe it.
"My agent called me and all I could say was 'oh'. I was just shaking, was really in shock. After that it all went very quickly.
“For two years I thought I would take my chance at Valencia, but I didn't have to hesitate when Atlético came forward. Or well, a little bit.
"I made very good friends in Valencia and had a great time there, but I still have the feeling that I can develop myself faster at Atlético."