KAZAN, Spain defender Dani Carvajal has been recalled for Wednesday's World Cup match against Iran, replacing Nacho from the starting side that drew with Portugal, after recovering from a hamstring injury.
Spain continued to work their way through the red maze and looked stronger in the second half, conjuring up as many shots as they did in the previous half.
Spain were camped in the Iran half for the most part of the opening 30 minutes, but they struggled to find openings in the final third.
Spain's Diego Costa (second left) is spoken to by referee Andres Cunha from Uruguay after appearing to stand on Iran goalkeeper Ali Beiranvand foot, who sits on the ground.
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.
Iran could have scored in the 80th minute but Mehdi Taremi's header went over the bar.
Iran started their World Cup campaign by winning over Morocco with 1-0.Whereas, Spain ended its first match with a 3-3 draw against Portugal.
After an exhilarating first six days, the seventh day of the World Cup will see some of the biggest players in action. We all know it's tough to play them.
"A World Cup match is completely different from all others, it's incredibly lovely", he confirmed.
But he was ruled off-side by the VAR, prompting howls of derision from white-clad Iranian fans. In the more likely situation that Spain secure victory, they will enter their final group encounter in a much more comfortable position for reaching the first knockout rounds.
Spain chalked up the 1-0 win over Iran at Kazan Arena thanks to a strike from Diego Costa. The Atletico Madrid striker showed his individual class and poaching instincts with both goals and will be charged with breaking the deadlock against a mean Iranian defence.
Costa has now scored nine goals in his last nine worldwide starts for Spain.
Best of worst moment of the 2018 World Cup thus far? He has scored three goals in two matches, but he can do even better.