Del Bosque left Costa out of Spain's squad for the upcoming friendlies against Italy and Romania with reports suggesting he was unhappy with the forward's discipline, but he insists his absence is due to a quadriceps injury.
Costa's on-field temperament came under scrutiny in Chelsea's recent FA Cup defeat at Everton, with the former Atletico Madrid star sent off late on in the contest following an altercation with Gareth Barry.
It was suggested after the match that Costa may have bitten his opponent, although both players subsequently denied that, while the 27-year-old was also cleared of making an inflammatory gesture to the home fans as he left the pitch at half-time.
And Del Bosque believes Costa is being treated unfairly, telling Mundo Deportivo: "From what I've seen, they have demonised Costa a lot and maybe that's why [there are some who don't want him in the squad] but the last episode isn't so serious, honestly.
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"He was putting in effort to play because it was Chelsea's last chance to get something and he plays with great excitement.
"That's a good part of his approach. He also has other things that are bad, but I don't want either to change.
"If we called him it's because we already know him, but I don't like that he does ugly things, but the other day wasn't like that."
It has been a season to forget for Premier League champions Chelsea, who are 10th with eight matches remaining and out of the Champions League and FA Cup.
But Del Bosque has no concerns over Costa's team-mates and fellow Spain internationals Cesc Fabregas, Pedro and Cesar Azpilicueta
"Cesc is not playing badly and Azpilicueta either," he added. "Pedro has had an injury and has not been all good, admittedly, but he has been quite reliable with us and I hope that it remains that way."