Iran had relied nearly entirely on their steely defence to keep them in the match for the first 54 minutes and it was only after the ball had deflected off Costa's knee and into the net that they took the game to the Spanish.
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.
That near-miss was brought into sharp focus less than 60 seconds later when Costa picked up Andres Iniesta's pass inside the area and saw Rezaeian's attempted clearance ricochet off his leg and past the helpless Beiranvand. Only one minute later, another shot from Spain was tipped away by Beiranvand.
Finally, in the 54th minute, Spain scored the goal which reflected their superiority, though the goal itself was fortuitous at best.
Spain put on a slick and efficient display against Portugal and Hierro is not expected to make major changes to his lineup. Incredibly, they mustered just one shot on target in the first half - despite enjoying more than 80 percent of possession.
"The truth is I had a bit of luck (with the goal)", said the Atletico Madrid star after the game. "I don't think that's football; it's not in the spirit of the game".
David de Gea was one of the players to receive criticism on the back of the 3-3 draw with Portugal, as his error gifted Cristiano Ronaldo his second of the game. Even taking into account the last-minute loss of their coach, Julen Lopetegui, who has already joined Real Madrid for the upcoming season, there's no clear reason behind such nerves in players who knew exactly what they had to do when they arrived in Russian Federation.
Iran has three points but Morocco has been eliminated.
They will have to score past the Spanish defence of Jordi Alba, Gerard Pique, Sergio Ramos and Nacho Fernandez will feature in defence.
"I have 23 players with their own character and you can't treat them all the same", Hierro said.
Iran went into the World Cup with few expectations but pulled off a shock in their opening game as they beat Morocco 1-0. And results have not been too bad, as Spain is level on points with Portugal for top spot in Group B and in an excellent position to qualify for the knockout stage. Aside from the goals, he's working really hard and helping us a lot.
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