19:00 BST, Monday 25th June, Mordovia Arena (Saransk, Russia), BBC
With four points from their opening two games -a 3-3 draw with Spain and a 1-0 win over Morocco - Portugal have one foot in the last-16 of the World Cup, though Iran are tightly gripping the other, holding them back from qualification.
Team Melli were very unfortunate to lose to Spain in their previous game, brilliantly holding them at the bay until Diego Costa's knee had its say, but their win over Morocco in their opener stands them in good stead to qualify, too.
Group B is extremely tight, but Iran must beat Portugal to seal their spot in the next round, as it is assumed Spain will beat Morocco. For the European champions, however, a point is enough to take them through.
Last Time Out
Iran 0-1 Spain
Despite a battling performance from Team Melli, the Iranians were ultimately undone by Diego Costa's knee, which proved to be enough to earn La Furia Roja their first victory of the World Cup.
A frustrating first half saw Spain dominate 81% of the ball, but Iran erected a formidable barrier which the Spanish found impossible to penetrate with their easy-on-the-eye passing triangles.
There were bodies flying everywhere in the penalty box and Iran defended gallantly, but they were eventually breached when the ball ricocheted off Costa in the 54th minute.
Carlos Queiroz's men then played more expansively and had chances of their own, with Saeid Ezatolahi's effort crucially ruled out for offside although Team Melli had believed they'd drawn level.
Despite surviving a desperate goalmouth scramble - with Spain unable to convert from a yard out - Mehdi Taremi headed one of their best chances over the bar as Iran suffered defeat.
Portugal 1-0 Morocco
Cristiano Ronaldo continued his brilliant start to the World Cup with his fourth goal of the tournament, coming from his fifth shot. His goal saw him become the highest European international goal scorer in history.
The 33-year-old has transformed himself into a ruthlessly efficient poacher, demonstrating as much with another goal in the fourth minute against Morocco, diving low to head powerfully from Joao Moutinho's cross.
Portugal regressed into themselves as the game wore on, as Fernando Santos instructed his men to retreat into their low block, but Morocco could not find a breakthrough - many of their best chances fell to Juventus centre back Mehdi Benatia - as they became the third side to be eliminated from the World Cup.
Queiroz made three changes between the Morocco and Spain games, and he's known to alter his XI based on the opponent, so he could do so again.
In need of a win, Iran may start in a more attacking system, with their 5-4-1 becoming a 4-1-4-1, and Omid Ebrahimi starting in a more advanced role. Alireza Jahanbakhsh will have to settle for a spot on the bench.
Portugal's weakness is their defence, specifically Raphael Guerreiro, who was poor again in the win over Morocco, which means Mario Rui could replace him, though he only has four caps to his name.
Goncalo Guedes, too, failed to impress, and Andre Silva could see himself reinstated to the starting XI, with Joao Mario likely to keep his place on the left flank over Bruno Fernandes.
Elsewhere, Portugal's team are settled and finding their rhythm, so there's little need to make excessive changes.
Key Battle: Sardar Azmoun (Iran) vs Jose Fonte (Portugal)
Against Spain, Iran had their backs to the wall for the majority, enjoying only 22% of possession, which makes the presence of a target man all the more important.
In the first half, when the ball was one back, it was thumped upfield to no-one in particular, though once they started countering after the break striker Sardar Azmoun became crucial. He provided a physical threat up front and gave Sergio Ramos and Gerard Pique something to think about.
Azmoun, moreover, held the ball up well and brought his teammates into play, providing a structure to the counter-attack and an outlet to relieve pressure.
KEY STAT: Sardar Azmoun won eight aerial duels, more than any other player.
This will be vital against Portugal, especially against an ageing Portuguese centre back pairing. Pepe is the better of the two and nuisance to play against, which is why Azmoun should target Jose Fonte for one-on-battles.
The West Ham centre back is declining as a footballer and Iran should look to put him under pressure in the air against Azmoun as a means of getting into the Portuguese box.
Which Iran will we see?
Carlos Queiroz is a very tactically astute manager, and everything his Iranian side do is for a highly specific reason.
He knew that a point against Spain would stand them in good stead for a spot in the knockout reasons, hence his decision to pack their penalty area with bodies and sit extremely deep behind the ball.
In the second half, however, once Costa had scored, a different Iran emerged. Their low block advanced and Team Melli countered at every opportunity, additionally beginning to press the Spanish defence in their own third of the pitch.
A point isn't going to be enough against Portugal, though, which means Iran will have to make an attempt to snatch a victory, most likely through counter-attacking football. They'll have to come out of their shells and play a little more expansively against a defensive Portugal side.
Shutting down Cristiano Ronaldo
This is what Queiroz has most likely been focussing on since the previous fixture: How to shut down Ronaldo. There was no one key player for Spain, rather the entire XI that needed managing, hence the 5-4-1 system that limited space and forced them wide, where they were arguably weaker.
With Ronaldo, however, the temptation is to double up on him and ensure there's always a spare man to cover if he manages to wriggle through.
This is to the neglect of the rest of the team, though, and there's certainly quality elsewhere in Bernardo Silva and Goncalo Guedes, for example. Doubling up on Ronaldo, too, will create space elsewhere at the back for others to exploit.
The decision, therefore, is to put the emphasis on retaining shape or deploying a plan with the intention of isolating Ronaldo.
Prediction: Iran 1-1 Portugal
Iran know that a victory will send them into the last-16, whilst Portugal, too, are aware of the fact that they'll be the ones to suffer, dropping out of the top two of Group B - provided Spain better their result.
It could mean both teams play expansively, hoping to get an early advantage, which could open up proceedings.
However, with both teams defensively orientated and seeking to encourage the other to be proactive and play on the front foot, this game has the potential to be rather cagey, ending in stalemate. 1-1.
Listen to the RealSport football writers discuss Portugal and Iran's previous games, plus the rest of the action, from Day 7 of the World Cup in Kremlins in the Basement: RealSport’s daily World Cup podcast.