The 27-year-old received the call from Vicente del Bosque after scoring 22 goals in his first season in England, helping the Welsh side finish in the top half of the Premier League and also win the League Cup.
The European and world champions take on Belarus and Georgia at the end of their qualifying campaign and know that wins will guarantee them a place in Brazil.
"For any player, to be in the national team is the best," Michu told reporters. "To wear the shirt of La Roja is to wear the shirt of the best team in the world.
"It's a dream, all children dream of this and I feel privileged. It is my greatest achievement. Am I nervous about my debut? You're always nervous, but when you start playing with players at this level, it makes things easier.
"I have to take this opportunity to enjoy the moment and help my teammates to close out qualification to the World Cup."
Michu scored 15 league goals for Rayo Vallecano the season before moving to the Liberty Stadium.