Having convinced Costa to turn his back on Brazil, the country of his birth, Del Bosque has so far shown plenty of belief in the forward and gave him a chance to shine at this year's World Cup, but he and Spain failed badly.
The team were unable to retain the trophy they won four years earlier, exiting at the group stage, and Costa was unable to score. He has now failed to register a goal in five games for the national team.
Costa will be the focal point in attack as a new look Spain, with veterans Xavi, Xabi Alonso and David Villa all having announced their international retirement, seek to win the European Championship for a third straight time.
They face Slovakia on Thursday and Luxembourg on Sunday as they look to progress in qualifying Group C that also includes Macedonia, Ukraine and Belarus.
Del Bosque's side got off to a good start with a 5-1 victory over Macedonia last month, which Costa missed through injury.

The 26-year-old striker came to the fore as he won La Liga with Atletico Madrid last season, notching up 36 goals, and has found no difficulty adapting to life at Chelsea, with nine in nine appearances so far.
But for Spain it has been very different and as well as his inability to score he has not shown the tigerish qualities that unsettle defences and wind up opposing fans.
"Diego has in front of him the challenge to show everyone that we have taken the right decision," Del Bosque said.
"Maybe we are wrong, and it does happen in football, but I have a lot of confidence in him that he is going to do very well,” he added.
"I think that he can be very important for us as I believe in his qualities. He has put his faith in us and we have put ours in him. If I had seen anything that made me doubt him then I would say it,” he further said.

Del Bosque does not believe Costa's failure to score is due to incompatibility with Spain's style of play, even though he likes to latch on to long balls from midfield, while La Roja play a shorter passing game.
"He is different to (Fernando) Llorente but he and (Paco) Alcacer are quite similar, they look for space and make runs to open the defence," Del Bosque said.
"In the last European Championships we had (David) Silva, Cesc (Fabregas) and (Andres) Iniesta. It would have been great if we could have had Diego along with them as well,” he added.
Costa did go to the World Cup in the wake of hamstring trouble, which clearly did not help his form, but he has been hampered by similar problems at Chelsea and been able to find the back of the net regularly.
In fact, he has been called up to the Spain squad with Jose Mourinho questioning his fitness, but the ever diplomatic Del Bosque looked to avoid any confrontation with the Chelsea coach over whether Costa is able to play.
"We all want the best for him and our teams. We are not going to push anyone too much and we are not obsessed over a single player," Del Bosque added.

Latest News from Sports News Desk