The Chelsea forward has left the national squad following their 1-0 defeat to France in order to receive treatment on a hamstring problem.
Head coach Vicente del Bosque opted to replace Costa with the uncapped Munir for the up-coming Euro 2016 qualifier with Macedonia, and the 19-year-old says he could not believe it when the call came.
"When they told me, my mouth fell open. It's a dream to play for the Spain team and I'm very happy," he told reporters.
"I've realised a dream. It's a moment I want to enjoy, an opportunity to take, and to give my best."
Munir, who has also been eligible to play for Morocco, says he would relish the chance to represent Spain at Euro 2016 in two years' time.
"Every trophy you can win is good and to be there would be a hugely proud moment, to play with the Spain team is what I would like to do," he added.
"I've followed the national team every summer that they have played since I was small, and now I want to thank my family and friends for supporting me at every turn."