The Kazan Arena is set to host would could prove to be a decisive match in Group B.
Iran head into this clash with Spain as the surprise leaders of the group after narrowly beating Morocco by a single goal to nil in the opening round of games.
A heartbreaking 95th-minute own goal from Morocco's Aziz Bouhaddouz handed Iran their first win in a World Cup finals since 1998, blowing the table wide open.
Spain, meanwhile, were held to a draw by a Cristiano Ronaldo-inspired Portugal in arguably the most entertaining game of the tournament so far.
Managerial chaos in La Roja's camp before the tournament started threw their preparations up in the air, but anything but a win on Wednesday will give leave the European giants' chances of progressing hanging by a thread.