Mauricio Pochettino criticised Tottenham for not showing enough respect to Southampton as they were beaten 2-1 by his former side at St Mary's on Saturday.
Though Spurs dominated early on and took the lead through Harry Kane, goals from Yan Valery and James Ward-Prowse mean Pochettino's side are without a win in four games in the Premier League.
Pochettino was in the stands for the clash, serving the first of a two-match touchline ban, and his overall perspective on the game was that his side threw it away.
As it happened at St Mary's
Match report
He told Sky Sports: "We didn't respect too much the opponent, and in football first of all you must first respect, then fight, then play football and show your better quality than the opponent.
"Hard to believe? No. The challenge that we are trying to develop, I think it is our challenge to have the momentum in the game. It's difficult to understand the second. We really stopped the fight, stopped being aggressive, we started to give the belief to the opponent, and that is the worst.
"When you are so much better than we show in the first 45 minutes, I think the second half it is difficult to understand why we changed a lot."
He added in the post-match press conference that Spurs showed an arrogance which is indicative of their mentality at times, something he admitted he is worried about.
"We played the first half like [one of] the best eight teams in Europe but in the second half we need to watch the game, we need to analyse and we need to be critics with ourselves," he added. "I am so, so, so disappointed.
"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."
Tottenham's top-four hopes, which seemed secured just weeks ago, are now under threat as we enter the final two months of the campaign.
Pochettino felt the setback on the south coast highlighted the club's deficiencies and provided a reality check in their quest to earn a place among Europe's elite.
"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.
"Everything you need to increase if you want to be one of the best clubs in the world.
"We need to be careful how we assess ourselves and it's so important to live in the reality. I think in the last few years we were living more in the perception than in the reality and now is the reality."