Del Bosque concedes that he should have looked at Michu earlier after being impressed in training.
"An international manager should try to be active picking some players up for the team," he told reporters.
"Sometimes we have to say sorry because we weren't active in calling Michu, because the first impression is very good.
"Maybe we haven't been active bringing him up but sometimes it depended on the circumstances because we have a lot of players."
Spain's game against Belarus is in doubt after a generator exploded at the stadium.