Iran coach Carlos Queiroz did not reveal the identity of the person, who felt ill after referee Andres Cunha disallowed Saeid Ezatolahi's goal.
Iran will take confidence from this defeat but Spain will be frustrated that their dominant possession can not be turned into goals, but for the moment there will be relief in the Spanish camp that they held on for three points.
Coming up against a determined Iran outfit, La Furia Roja got the victor nine minutes after halftime when Diego Costa found the bottom corner via a deflection.
"We were not lying yesterday when we said in the press conference Iran is a tough team", said Spain coach Fernando Hierro, who replaced the sacked Julen Lopetegui only two days before their opening match against Portugal.
Another chance went out the window as a late header from Mehdi Taremi went just over David De Gea's crossbar after Vahid Amiri had nutmegged Barcelona star Gerard Pique. Said Queiroz: "We deserved a better result".
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"We are a team that plays direct, open and I hope we do not have rivals that put many players in their area".
"It's a crucial match that will determine our World Cup destiny", said Isco.
Spain are overwhelming favourites to take all three points from the game, but Iran's defensive record could ensure their task in Kazan is far more complicated than the bookmakers are suggesting.
Iran started the second round of fixtures top of Group B having beaten Morocco in their opening game, while Spain needed a win after a thrilling 3-3 tie with Portugal.
Diego Costa bagged his third goal at the 2018 World Cup as Spain defeated Iran 1-0 in their second Group B encounter on Wednesday night. It appeared that Beiranvand exaggerated his pain after Costa stood on his foot. "They were provoking us, and we were on the grass all the time".
The Real Madrid defender was only playing because his club-mate and first-choice right-back Dani Carvajal was injured, but he came close to writing himself into World Cup history by thrashing home a spectacular half-volley from the edge of the box to give Spain a 3-2 lead.