Days after celebrating only their second-ever World Cup win in five editions with a 1-0 victory over Morocco that sent them top of Group B, Iran are expected to return to earth with a bump against a Spain side oozing class.
Costa's goal came in the 54th minute as a defender tried to kick the ball away.
"We started well, but then we lost our rhythm because of frequent pauses".
Iran did not concede a single goal until the last of their 10 final round Asian qualifiers when their spot in Russian Federation was already secure, at the same time creating enough going forward to win six of those matches.
"When I speak about my team, I speak about the 23 players", Hierro said. It will be a very hard tournament.
Iran sits in third place in the group with three points while Morocco is in last with two losses.
Iniesta, who scored Spain's extra-time victor on their way to victory over the Netherlands in South Africa in 2010, is playing his final World Cup having recently ended his long and successful club career with Barcelona to join Japanese side Vissel Kobe. His relentless work to win the ball back and his dribbles also contributed to wear down the opposition on a frustrating night for Spain's attacking players.
This goal took Costa's Spain total up to 10 in 22 matches, with nine of them scored in his past 11 outings.
But Hierro insists the problems posed by Iran - who thought they had equalised only for Saeid Ezatolahi's effort to be ruled out by the video assistant referee - indicate there can be no guarantees in Russian Federation. "If we had six more months (without it) maybe we would have had a point - but it is good for the credibility of football".
And Hierro - in charge after Julen Lopetegui's shock sacking - accepted he actually felt happier after the draw with Portugal than after seeing his side beat Iran. "We just hope things happen the way we want".
After a match played out to a deafening blare from vuvuzela horns from a huge army of Iranian fans, Spain will feel relieved to have emerged with maximum points.
Diego Costa (1st L) of Spain celebrates scoring during a Group B match between Spain and Iran at the 2018 FIFA World Cup in Kazan, Russia, June 20, 2018.
"We go for it, with great belief and great determination to try to win the game", the master tactician told reporters.
"It's really a unique opportunity for us to play against Spain".
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