Barcelona had a lucky escape when deciding not give Marlon Santos his LaLiga debut, as it could've meant being handed an automatic defeat.
It also forced Barça into not including the player in their squad to face Hercules in the Copa del Rey on Wednesday.
A bureaucratic problem arose at the last minute and with the Brazilian centre-back set to make the trip to Alicante with the first team, the federation advised that doing so could have serious consequences.
Marlon's case refers to the section of RFEF's rules which state: "Players registered in the reserve team can only line up for different teams to the one they are registered with if their registration with the reserve team was processed within the inscription period."
Barca did register the player, but not fully as several documents were left outstanding - when it was completed it was after the specific deadline date.
The defender was able to feature against Celtic in the Champions League due to different rules with UEFA.
Marlon has featured regularly for Barca B this season.