Michu had been tipped to make his international bow in Spain's friendly against Uruguay next week but he has been overlooked from Del Bosque's 21-man squad.
Del Bosque told reporters: "It's left a little bit of a bad taste in the mouth not to call up someone who deserves it. But those who have been named also deserve it.
"We recognise his worth and his goals in the Premier League but we couldn't find a space for him at the moment.
"The same thing happened with Iago Aspas. I'm honest and don't hide anything. In the position which Michu plays there is great competition with five or six players."
Michu has scored 16 goals for Swansea this season.