Spain will look to ease pressure on them with their first win of the ongoing FIFA World Cup, while Iran will aim to register their second consecutive triumph in the Group B when the two sides clash here on Wednesday.
Spain tied 3-3 with Portugal on Friday in their first international match under newly-minted head coach Hierro, who took the job just two days before the World Cup after Julen Lopetegui was sacked for announcing he would lead Real Madrid next season.
For 2010 World Cup champion Spain, the draw had felt more like a defeat, especially given the team's inspired performance in spite of a tumultuous few days marked by their abrupt change in coaches on the eve of the tournament.
Spain and Portugal are tied for second in Group B with one point each, while Iran lead the group with three points after beating Morocco 1-0 earlier Friday, thanks to an own goal from Aziz Bouhaddouz.
An Iran win, though improbable on paper, will take them to the Round-of-16, while weakening the 2010 edition winners' chances of progressing to the next round.
However, it will be yet another big test for Iran. A strong defence is Iran's main strength. It will be interesting to see whether captain and central defender Jalal Hosseini and right back Steven Beitashour return to the line-up with Roozbeh Cheshmi, Morteza Pouraliganji, Ehsan HajiSafi and Ramin Rezaeian being preferred on Friday.
Masoud Shojaei, Omid Ebrahimi, and Vahid Amiri are expected to start in midfield, while the attack will be led by Sardar Azmoun, who plays for Russian club Rubin Kazan. Azmoun will be flanked by Karim Ansarifard of Greek club Olympiacos and AZ Alkmaar's Alireza Jahanbakhsh, who was Dutch league's top scorer last season.
They will have to score past the Spanish defense of Jordi Alba, Gerard Pique, Sergio Ramos and Nacho Fernandez will feature in defence. David de Gea, who had a poor performance against Portugal, is expected to start in goal.
Depending on the number of forwards Hierro chooses, Spain's midfield may rotate in a three or four-man central players. Koke, Andres Iniesta, Sergio Busquets and Isco might feature, while in attack David Silva and Diego Costa, who scored twice in the opener, are certain to start. Hierro may use the pace and mobility of Iago Aspas and Rodrigo Moreno as Iran will look to make a heavy defending duty.