Real Madrid's status in the Copa del Rey could be under threat after they fielded Denis Cheryshev in the first leg of their last-32 tie against Cadiz.
The Russian winger was booked three times in last season's competition when he was on loan at Villarreal, including in the second leg of their semi-final defeat against Barcelona.
That triggered a one-match suspension that has not been served, yet Cheryshev started against Cadiz on Wednesday, scoring after only three minutes of the match.
Amid confusion, the 24-year-old took to the field for the second half, only to be replaced within 40 seconds of the restart by under-pressure head coach Rafael Benitez.
And Jorge Perez of the Royal Spanish Football Federation (RFEF) told Cope: "There was no amnesty last summer and we are going to check files to see if Cheryshev was banned."
Meanwhile, Cadiz confirmed their directors would hold a meeting after full-time to discuss their next move.
Fran Canal, a director at the Segunda Division B club, told Canal Plus: "We are going to wait until the game is over. We need to complete the formalities and will have a meeting with the board.
"We will respect Real Madrid, but our fans and our members always want the best for Cadiz. If there is a chance we will report [Real for picking Cheryshev].
"I have checked all the banned players that the Spanish FA reports at the beginning of the season and this player had a one-game ban.
"Considering this is the first Copa del Rey game this season with Real Madrid, he could not have played. I insist [we will] not rush on this. From Cadiz we want respect for Real Madrid.
"You can have no doubt. If there is an improper alignment, we will make de report."
"Even if he had played one single minute, it would count as well. The result does not matter at all."
Osasuna were eliminated from the Copa earlier in this season's competition for fielding an ineligible player.
Madrid have already had a turbulent start to the season, with president Florentino Perez forced to give Benitez a public vote of confidence following the recent 4-0 home defeat to Barcelona in El Clasico.
Barca defender Gerard Pique appeared to poke fun at the apparent blunder by posting a series of crying and laughing emoticons on his personal Twitter page during the match.
The incident will provoke an uncomfortable reminder of a similar mishap for Benitez, whose Valencia side were expelled from the Copa del Rey in October 2001 for selecting four non-EU players in their matchday squad for a 1-0 win over Novelda.