Costa was left out of Spain's squad for the upcoming friendlies against Italy and Romania with reports suggesting Vicente del Bosque was unhappy with the forward's discipline, but the Spain coach clarified on Tuesday the striker's absence is due to a quadriceps injury.
Garcia has little doubt the Chelsea striker will be back for the Euros, with Valencia's Paco Alcacer and Atletico Madrid forward Fernando Torres as his understudies as they chase European glory again.
"I think Diego Costa is the man for Spain, yes," the former Liverpool winger told Omnisport.
"If he finishes the way he does with [Cesc] Fabregas, the way he does right now at Chelsea, then he's going to be our striker with [Paco] Alcacer and maybe [Fernando] Torres, or maybe another player who is coming through right now."
The 37-year-old believes Costa will not be the only key figure for Spain in France, saying Real Madrid's Isco could also make the difference for Del Bosque's side.
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"I think Isco is going to be one of our main men; with [Andres] Iniesta, with Santi Cazorla - hopefully he gets fit – [David] Silva of course, another player [Sergio] Busquets, the midfield is going to be very strong," the ex-Liverpool man added.
"And for me one of the most important players is going to be Isco for the national team because he's got something different.
"It's not about passing, or it's not about pace, it's about having something different and he's going to probably be the key player to break some of the defenders of the other national teams."
And Garcia expects Spain to bounce back from their disappointing World Cup performance in France, and has backed them to lift a third successive title.
"We can win it, definitely, yes. It's about the approach you have. I think they have been doing well," he said.
"They've been winning games and then the confidence came back again after what happened during the last World Cup. Of course everybody was expecting Spain to be there for the semi-finals and then the final but they were so consistent for the last eight years, so it was difficult. No-one had done it before to stay there for so long.
"Spain, I think, are ready. The team Vicente del Bosque has got, I think that confidence was missing during the last World Cup and hopefully, like I said, the approach goes well and they can fight for the trophy."