Mauricio Pochettino accused Tottenham players of not taking the second half seriously after conceding twice in five minutes at Southampton, a 2-1 defeat that threatens their Champions League qualification for next season.
Days after securing a place in the season's quarterfinals, third-placed Spurs face a battle to hang on to a top-four place in domestic competition and Spurs manager Pochettino -- serving the first of a two-match touchline ban -- questioned his players' mentality.
"We need to realise if you do not fight with intensity for the Champions League, you will struggle," said Pochettino, whose side appeared to be cruising at St Mary's after Harry Kane scored his 200th career goal on 26 minutes.
But Saints revived after the interval, battling to an unexpected three points.
"The reality is you cannot only play for 45 minutes, we are a Champions League team, into the quarterfinals," said Pochettino, whose side beat high-flying Borussia Dortmund 1-0 in Germany on Tuesday to complete a 4-0 aggregate win.
"When you lose the momentum after 45 minutes, that is about mentality. For us, we lost the momentum and gave them relief. It is so difficult to understand why we changed so much from the first half to the second," Pochettino added.
"I am very disappointed, we were better in everything and then we lose the game.
"It is about competing better for the first goal if you want to be a contender for big things. The second half we did not take it seriously."
For much of this season a top-four place looked assured but three defeats and one draw in four games -- their worst league run for 15 months -- has left Spurs three points ahead of fourth-placed Manchester United, with Arsenal and Chelsea close behind.
"We are fighting against big sides [for the top four]," said Pochettino. "We had a big gap, welcome to the reality, we are under pressure to win games. It is a massive challenge for us."
Tottenham have three weeks to prepare for their next league game at Anfield against a Liverpool team still firmly in the title chase.
Pochettino's future at Spurs continues to be the subject of debate, having told Spanish outlet RTVE on Friday that: "I would love to return [to Spain] one day and oversee a brand of football that could win games and excite people."
The ex-Espanyol player has been one of the names tied to a possible move to Real Madrid, who could soon sack embattled boss Santiago Solari. Former Blancos managers Jose Mourinho and Zinedine Zidane are also in the mix to return to the Bernabeu.
Information from Reuters was used in this report.