Mauricio Pochettino believes Juan Foyth's ineligibility for Tottenham's Champions League campaign shows they cannot afford to repeat the 'painful mistakes' of his side's summer transfer activity.
The Argentinian has forced his way into Pochettino's side in recent weeks, starting the last three Premier League games, but will miss the crunch Champions League clash with Inter Milan on Wednesday as he is not included in the club's 25-man squad.
The 20-year-old defender was omitted from the list as Spurs have 20 "non-locally trained" players in their squad and are only allowed to name 17, so he was one to be sacrificed.
Spurs' inability to move any of their foreign players out in turn meant they had to target homegrown talent, which saw them become the first team in Premier League history not to sign anyone during the summer transfer window.
Pochettino said: "I'm so disappointed from the beginning because it's not only him but Georges-Kevin N'Koudou and Vincent Janssen are out of the squad in the Champions League and that's so painful.
"It's so painful. It's a mistake that we made that at this level of football we cannot repeat again. For different reasons we need to find solutions before the start of the season.
"I was so disappointed because the squad was so big and when the people say, 'You're the only club that doesn't sign any players', to sign another player means to put another player out of the list for the Champions League.
"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."
Whether or not Spurs are in the competition after Christmas remains to be seen as they host Inter in a must-win Group B game.
Pochettino's side will go out if they fail to beat the Italian side at Wembley.
The Argentinian is hoping Dele Alli can step up to the plate again, given his knack for producing the goods in big games.
The England international scored in the impressive win against Chelsea on Saturday and Pochettino says he gets bored unless he is playing under pressure.
"You know Dele, he loves a challenge, every single game," he said.
"He is a winner, his mentality is that he gets bored quick when he doesn't feel the pressure and challenge.
"We laugh a lot when he is alone with the ball in front of the goal he is boring, we say, 'Oh that is Dele Alli'.
"But when he is with opposition and with pressure he is unbelievable. He is a competitor, he loves this type of game, he loves to compete in a difficult situation."
On his side's chances of progression, which is also highly likely to require a win in Barcelona next month, he added: "It is going to give us hope or give us a different reality that we cannot fight to get through.
"It is a decisive game, it is a must-win game.
"When people say something is going to be difficult I am so excited to show it is possible to do. We need to win the two games. We are going to try."