The form of the 24-year-old has been a key factor in the Blues moving to the top of the Premier League table.
And the former Valencia man also scored in the final of Euro 2012 as Spain crushed Italy 4-0 in Kiev in July.
But coach Vicente Del Bosque has not selected the player for the forthcoming World Cup qualifiers against Belarus and France - and Mata's disappointment is obvious.
"I was excited about these games and confident I would make the squad," he said, quoted in The Sun.
"I've not spoken to the manager. The only thing I can do is keep playing well in order for him to pick me."