Real Madrid president Florentino Perez insists the club will battle any sanctions for fielding an ineligible player in the Copa del Rey victory at Cadiz on Wednesday night.
Russia winger Denis Cheryshev started in the fourth-round tie despite being suspended after picking up a third yellow card in the semi-finals of last season's competition when on loan at Villarreal and scored Real's first goal.
Cheryshev was withdrawn from the action moments after half-time when Real had realised their error, with head coach Rafael Benitez saying they wanted to "show good faith".
Last season Osasuna were kicked out of the tournament for playing an ineligible player while Benitez was in charge of Valencia when they were disqualified from the Copa del Rey having played too many non-EU players in a tie in 2001.
But Perez believes that as neither the player nor the club were informed of the suspension that Real cannot be kicked out of the competition.
Perez said at a press conference on Thursday evening: "Nobody told the club or the player, not the federation nor Villarreal, so it was impossible for us to act differently. It's not our fault.
"What happened last night has never happened to Real Madrid before. If we didn't know, the sanction cannot stand.
"We checked the situations of all of the players listed in the squad rigorously before yesterday's game. Look, in the football section, there is a department dedicated solely to these kinds of issues; no-one told them about Cheryshev.
"Sanctions cannot be enforced unless the player is personally notified, that's the way I understand it."
Earlier, Cheryshev confirmed he did not receive notification from his former club that he would be banned.
He told the Real Madrid website: "I did not receive any notification from Villarreal to inform me that I had been banned from playing in the Copa del Rey."
It has been reported in Spain that a decision on the outcome is expected on Friday.
Speaking after the game, Real manager Rafa Benitez said: "Clearly, the club did not know anything," he said. "We weren't told by Villarreal or the RFEF. We knew nothing and [after finding out], we have substituted the player to show good faith."
And Real director Emilio Butragueno told Marca: "We knew nothing of the situation."
There is a precedent for such a situation in Spanish football, as Sky Sports' Spanish football expert Guillem Balague told Sky Sports News HQ: "The same thing happened to Osasuna in September, for fielding an ineligible player. They were kicked out of the competition as well."
Real can appeal any judgement from the league but it is unlikely they will succeed, according to Balague.
He said: "We actually have a match delegate who is responsible for the line-ups and the referees, and actually checking if there are any suspensions, so I'm sure he'll be blamed.
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