The Atletico Madrid star was included in Spain's squad for friendlies against Equatorial Guinea and South Africa.
Costa, who rejected advances from the country of his birth Brazil, can only make the world and European champions better, according to Del Bosque.
"Each player is different and I have called those who are complementary. Diego Costa meets all the requirements," Del Bosque told reporters on Thursday.
"He's playing very well and is in a good moment. We have seen fit to include him because he is an option. He can only add to this team.
"We asked about his situation. We saw that we could play him and built on that following a logical process. It all started in September. He meets all the requirements.
"We met him personally and he gave us a tremendous feeling. His credit is not unlimited. From here to the World Cup there is a long time."
Despite often playing without a recognised striker, Del Bosque has no doubts the addition of Costa will work for Spain.
"Our style with Diego Costa in the team? We have smart players and they are able to adapt to the characteristics of the new players," he said.
"The adaptation will be good. Nobody is harmful for this team. A player like him, fighting with his energy and quality with and without the ball – why would that be negative for our game?
"We have midfielders who understand the game very well and Diego Costa will present many options. I am sure he can play with (David) Villa. They already do on their (club) team."
Xabi Alonso has returned from injury to be included in the squad, while goalkeeper Iker Casillas has kept his place.
