The 23-year-old, who could make his full debut against Finland on Friday, has played six times for the Under-21 side.
"I was at home with my family when I heard. I only heard because I was bombarded with messages and whatsapps. It really was the last thing to cross my mind," he told reporters at a press conference.
"I was so surprised. I didn't believe it when I heard. It's been a gift from heaven. I haven't spoken with [Vicente] Del Bosque yet. But I am grateful for the confidence he has shown in me.
Del Bosque praised the defender when questioned on his inclusion on Tuesday.