The midfielder has also praised Michu and Alberto Moreno following their first senior call-ups to La Roja and has warned his teammates to savor life with the national team.
The 25-year-old, Brazil-born Costa recently affirmed his desire to represent La Roja at the senior level after gaining Spanish citizenship, although his decision is yet to be ratified by FIFA. Iniesta believes Costa would be deserving of a place in Vicente del Bosque's side should he be given the chance.
"If Diego Costa can be here and the coach deems it important ... as a player, he has shown almost unbeatable performance levels," Iniesta told Cadena SER. "If he comes here, he will be one more player. That has to be above all. The performance of a player is what gets you on the team."
Although Costa was unavailable for Spain's upcoming World Cup qualifiers with Belarus and Georgia, Del Bosque did hand a first senior call-up to Swansea attacker Michu and Sevilla defender Alberto Moreno, Iniesta said both have earned their way.
"I think they've found a very good group to come into and feel comfortable," Iniesta said. "Their performances are what have got them here and they can enjoy these matches. When you're here, it's because you have earned it. We'll try to make them feel as comfortable as possible."
Spain can secure its place at next summer's World Cup with four points from its final two games against Belarus and Georgia, and Iniesta feels his compatriots should relish the opportunity to defend their global crown after losing out to host Brazil in the Confederations Cup this summer.
"We have to take the final step for Brazil. Things have gone very well, but we cannot claim victory until we have done so," said the 29-year-old. "Everyone wants to return to Brazil. We'll try to defend our title.
"Concentration is normal, as it has always been. The national team is enjoying a unique moment, one which we have to enjoy and savor. There are very exciting challenges ahead of us."