Tottenham boss Mauricio Pochettino accused his side of “arrogance” and “complacency” after a 2-1 loss at Southampton, which increased pressure on finishing in the top four.
Third-placed Spurs are without a win in their last four league games and are four points ahead of fifth-placed Arsenal, having played a game more.
Manchester United can draw level with Spurs if they win at Arsenal on Sunday.
“I am so, so, so disappointed,” said Pochettino, who sat in the stands.
“I am a little bit worried about this change from the first half to the second half – it’s only one thing, it’s mental.
“It’s about complacency, it’s about arrogance in a bad way. You can use the first half as an example of [positive] arrogance, playing with intention, focus, concentration – we matched them in everything and we were much better.
“And in the second half it’s the arrogance in a bad way. We need to blame all of us, the club.”
Speaking to BBC Match of the Day, the Argentine, who was serving the first of a two-match touchline ban, added: “We were better at everything but we lost the game. We did not take the second half seriously.”
Pochettino’s side led at the break through Harry Kane’s 200th career goal, but Yan Valery and James Ward-Prowse scored in the space of five minutes to complete the turnaround.
Spurs, who reached the Champions League last eight this week with an aggregate win over Borussia Dortmund, are nine points adrift of second-placed Liverpool (who have a game in hand) and eleven behind league leaders Manchester City.
But Manchester United, who were 13 points behind Spurs when Jose Mourinho was sacked as manager on 18 December, can reach the same points total as Tottenham with a win at Arsenal on Sunday.
And Chelsea, who are five points adrift of Spurs, can overtake their London rivals if they win their two games in hand.
“I think it’s good now [that we’re in a battle for the top four] that people now are going to realise our real level and maybe stop with the perception,” he continued.
“That showed that we still have a lot of work to do, that showed we are not mature enough, that showed that to be in the last level you need more effort, you need more quality, to increase your capacity of commitment.
“We are fighting against big sides. We had a big gap – welcome to the reality – and we are under pressure to win games. It is a massive challenge for us.”
Tottenham have now not won a game in the Premier League since they beat Leicester nearly a month ago.
But they announced this week that they are finally set to moved into their new stadium at the start of April.