However, Mata is expected to be one of the first names on the teamsheet since joining United for £37.1m.
"This was important for the World Cup," Mata said in a press conference. "The Spanish squad is difficult to get into with a lot of players fighting for positions.
"So I hope it's going to be a great opportunity for me to show the manager of the national team I want to be there and have to be there.
"Now the moment is here, I want to play as much as I can and if things go good in the last part of the season the World Cup will be closer for me."
Mata has been handed the number eight shirt.