Tottenham manager Mauricio Pochettino says it is "painful" that Juan Foyth is ineligible to play in their must-win Champions League game with Inter Milan.
The 20-year-old was not named in Spurs' squad for the competition but has impressed during an injury crisis.
"It's a mistake we made, that at this level of football we cannot repeat," Pochettino said.
The Argentina defender has started Spurs' last three games in the absence of Davinson Sanchez and Jan Vertonghen.
However, he was omitted from the 25-man Champions League squad because the club has 20 'non-locally trained' players in their squad and are only allowed to name 17.
"For different reasons we need to find solutions before the start of the season," he added.
"I told you we need English players, but it was impossible. Of course now that is the problem and it's so painful to have players out of the list.
"Now we're going to finish [the group stage] and then we hope in March we can do a new list and include him in the list for the Champions League."
Tottenham are third in Group B, three points behind Inter Milan, and must beat the Italians to keep their qualification hopes alive before heading to Barcelona in their final game.
Goalkeeper Hugo Lloris returns for Tottenham after serving a suspension following his red card at PSV Eindhoven last month.
With Foyth unavailable, Vertonghen, who returned as a substitute against Chelsea on Saturday, is in contention to start.
Danny Rose also comes into the Tottenham squad after recovering from a groin injury that has kept him out since the start of October.
But Kieran Trippier (groin),Sanchez (hamstring),Mousa Dembele (ankle) and Victor Wanyama (knee) all remain out.