Iran held their own in the first half as Spain failed to break down Carlos Queiroz's side, but Hierro insists that he was never expecting an easy turnover.
"Nobody is winning easily at the World Cup, we've had it as hard as we thought," he told reporters in a post-match interview with Telecinco.
"It's difficult to get into a rhythm with so much interruption. At half time we corrected some things and improved after the goal.
"Equality is at its maximum in this World Cup. Iran are a great team, with clear ideas. Queiroz has been working with them for seven years and they won't make things easy for anyone."
Spain, who dropped two points late on in a thrilling 3-3 draw with Portugal on Friday, needed the win to go level with their Iberian neighbours who beat Morocco on Wednesday afternoon.