The kickoff of the second half, however, saw the Spanish side pick of momentum in their chase for a goal, creating a couple of chances.
Iran beat Morocco, who are now eliminated, in their opener and will be aiming to produce a stunning upset in their final match.
Spain want to fully concentrate on their upcoming game against Iran despite recent turbulences following the change of head coach, coach and players here said on Tuesday.
Spain scraped a lucky 1-0 win over impressive Iran thanks to a deflected Diego Costa goal, as the equaliser was ruled offside with VAR. Do not cut, copy or lift any content from this website without our consent.
Spain close out group play against a Morocco side still looking for their first point of the tournament. The powerful striker turned in the area and took a shot but the ball deflected off Ramin Rezeian before bouncing back onto Costa's knee and into the net.
"We only care about sporting matters", he said.
Hierro said in the post-match press conference: "I think there are 32 great teams here at this World Cup and no-one is giving anything away, so anyone who thinks it is going to be very easy to score and win by a landslide is wrong".
"The Iranian team are very strong, both in defending and attacking".
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.
Iran will be without defender Roozbeh Cheshmi, who will miss the remainder of the World Cup after picking up an injury during training.
Against that, for all Spain's world class talent, Hierro has had little over a week to work with his team after being rushed into the job as a makeshift replacement after the sacking of Julen Lopetegui. He also has nine goals in his last nine starts for Spain.
According to the Twitter account of Tara Sepehri Far - quoting Iranian news sources - at one point the screening had been canceled.
Coach Carlos Queiroz concedes Iran now face one of the biggest challenges in their World Cup history, but maintains they can "make the impossible possible". "We're a team and we have to stick together". Portugal and Iran are also yet to play each other.