Dele Alli's tackle on Gent midfielder Brecht Dejaegere was a potentially career-ending challenge, according to former Tottenham manager Harry Redknapp.
Alli was sent off as Spurs were held to a 2-2 draw against Gent at Wembley, which meant they lost the tie 3-2 on aggregate and crashed out in the Europa League last 32.
Harry Kane's own goal had cancelled out Christian Eriksen's early opener before Alli's moment of madness came in the 39th minute.
The 20-year-old, frustrated at being denied a free-kick, jumped to his feet, miscontrolled the ball and then thrust his right foot high and into the shin of Dejaegere.
Dejaegere was able to play on but Redknapp, who coached Tottenham between 2008 and 2012, believes Alli was lucky not to cause him serious damage.
"[Alli] could have finished a fella's career. He could have finished a boy's career. There's no doubt. It was that bad," Redknapp said on BT Sport.
"He could have broken the lad's leg, snapped his leg, and that would have been the end.
"He's got to cut that out of his game. He's got that streak in him. I rave about the boy but that tackle is horrendous."
Alli's challenge was also criticised by former Tottenham midfielder and manager Glenn Hoddle.
"It's undefendable," Hoddle said. "You can't defend that. It was a reaction, a poor reaction to losing the ball, not getting the foul. You can't do that on a football pitch. Not nowadays."
It meant Tottenham were left needing to score twice more with 10 men and while Victor Wanyama gave the hosts temporary hope in the second half, Jeremy Perbet's late goal sealed Gent a surprise victory.
Alli has fallen foul of the law before.
He was lucky to escape punishment for kicking out at Fiorentina defender Nenad Tomovic in the Europa League last year and was then banned for three matches in April after punching West Brom's Claudio Yacob in the stomach.
Pochettino, however, defended his player's latest offence.
"He is very disappointed and sad," Pochettino said.
"It's difficult. It's not to justify him but it's a different action, he was fouled before, it was an action that was a clear foul.
"That's not to blame the referee but the action came in very soon, that tackle, and I think he deserved to be sent off. He knows. But that kind of action can always happen.
"It's only one and a half years he is playing for Tottenham in the Premier League, all that he achieved in a very short period is because he is special and a special boy.
"He has unbelievable personality and character. But before that he is a great player.
"In the same way, he is how he is. He is very passionate to play football and sometimes this action happens. Now it's time to support him, he is very sad and disappointed in the changing room."
Tottenham now turn their attention to hosting Stoke on Sunday and reducing a 10-point gap behind Premier League leaders Chelsea.
"It's key now to recover physically and mentally," Pochettino said. "Now is not the time to cry, it is time to move on."