If anyone was going to breach the solid defensive wall so carefully constructed by the Iranians, it had to be the in-form Costa, even if his goal had more than a touch of good fortune about it, as an attempted clearance bounced from his knee and into the back of the net.
Hierro has been hired to lead Spain at the World Cup as a replacement for Julen Lopetegui, who was sacked two days before the national team's opening match in Russian Federation.
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.
Iran remain on three points, while Portugal are one ahead on four and level with Spain at the top of the group after victory over the Moroccans in Moscow yesterday.
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. "We happened to meet at Malaga, then in the national team".
"Most of us have been playing together for many years and we know each other very well and we trust each other blindly". Iran was also risky on the break, using the speed of its attacking players to threaten the Spanish back four but without creating many real chances.
David Silva sent a free kick straight into the arms of keeper Ali Beiranvand in the 25th minute for Spain's first shot on goal.
"We are not just a team, we're a nation, a family, and I believe we have already shown in hard situations that we are usually united and able to achieve the best result".
Spain could not managed to get a second despite their best efforts while Iran nearly scored an equaliser when Mehdi's powerful header hit the crossbar. We are still alive and still dreaming'.
Should Iran pull off their biggest shock since 1998, they will qualify for the World Cup knockout rounds for the very first time and put Spain's campaign on the rocks.
Ultimately, Spain should have too much quality for Carlos Queiroz's side, and if they can call on any of the attacking prowess they showed against Portugal in their opening game, they should be comfortable winners.
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.
Iran did not manage a shot on target for the first time in 14 World Cup games. There haven't been many changes because he knows that we work well with what we already have. Iran lost a closely-fought game 1-0.
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To speak of England winning the World Cup had become slightly preposterous, and the preserve only of the more fanatical patriot. Stones then grabbed his second goal of the game with a header from close range after an impressively rehearsed set-play routine.