Iran began their campaign with a 1-0 win against Morocco but Queiroz has admitted they will have to produce their "best game ever" if they are to fulfil their aim of causing an almighty World Cup upset.
After a goalless first half in Kazan in which the Iranians frustrated Spain, the resistance was finally broken on 54 minutes through a fortuitous goal from Diego Costa, when the ball bounced back off Ramin Rezaeian, into the Spain striker and on into the net.
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's approach was far more positive following Costa's strike, Mehdi Taremi heading just over from close range in what was their clearest opportunity to find an unlikely leveller.
With three points from two matches, Iran still have a chance of qualifying if they can get a result against Cristiano Ronaldo's Portugal on Monday.
Fernando Hierro's side had to wait until the 25th minute for their first attempt on goal, David Silva's long-range free-kick comfortably held by Alireza Beiranvand.
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Spain resumed in more positive fashion after the break, Beiranvand forced to produce heroics twice in the space of four minutes.
Iran famously picked up the victory in the World Cup for the first time since 1998 against Morocco.
But he was ruled off-side by the VAR, prompting howls of derision from white-clad Iranian fans.
A scrappy second-half goal by Diego Costa broke a Persian stonewall and lifted Spain to a 1-0 win over a courageous Iran in a nerve-jangling World Cup clash on Wednesday.
Not an impressive result for Spain, who didn't look almost as sharp in attack as they did in the 3-3 draw against Portugal to open the tournament.
The FIFA World Cup 2018 Iran vs Spain match will be played at the Kazan Arena, Russia.
Time and time again we've seen goalkeepers come off their lines time in a bid to be their team's hero and score the goal they so desperately need.