Del Bosque opted to omit Chelsea striker Costa from his squad for the upcoming Euro 2016 qualifier with Belarus and friendly with world champions Germany due to a persistent hamstring injury - an issue that has led to a club-versus-country row over the last two months.
Costa's club-mate Fabregas also withdrew with a hamstring complaint, despite playing 90 minutes in Chelsea's 2-1 victory at Liverpool before the international break.
Spain defender Sergio Ramos told Radio Marca earlier this week that players should show the "same commitment to the national team as they do to their clubs", although he insists he was making a general point and not speaking about specific individuals.
And Del Bosque has now attempted to draw a line over the affair by stating his confidence in the honesty of his players.
"All the world can have their opinions but we have to make decisions," he told Radio Nacional in Spain.
"We are in an era of trust in some players who have given us so much. I have no reason to distrust them.
"If players don't come here because they're unfit, they're unfit. I have no reason not to trust either of them. There's no desire to harm anyone.
"Our authority is demonstrated in other things, not only leaving out or calling up the players.
"I have the utmost confidence in the players. I'm not fooling myself by saying a player is unfit when they're not."
