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 RFEF (Spanish FA) 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.
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.