‘All of it is disappointing. It’s the worst defeat since I’ve been here,’ said Postecoglou.
‘That’s probably the most disappointing loss I’ve had in terms of the way we went about it and it’s something I need to assess.
'We kind of accepted our fate and it's hard to understand as we've not done that while I've been here,’ added the Tottenham boss.
Spurs looked to be on their way to a sixth successive victory in all competitions after an electric first half display that saw them take a two-goal lead but were left stunned by three Brighton goals in 18 minutes after the break. And Postecoglou admitted that his side failed to do the basics.
Ange Postecoglou seethed as he slammed his Tottenham side after their defeat against Brighton on Sunday
‘We didn't do what you need to do at this level. it's non-negotiable - we just weren't competitive,’ he said. ‘We didn't win our duels, we lacked intensity and we didn't deliver the things you need to in terms of the basics and paid the price for it.'
Asked whether Spurs were guilty of complacency, Postecoglou wasn’t sure but stressed that the manner in which his side lost was ‘unacceptable.’
‘I’m not really sure (if it’s complacency) but wherever it comes from, it doesn't matter. It's irrelevant because it's unacceptable. You can understand that you're not going to win every game.
'But there's the manner you lose games in. And that's the first time since I've been here that we've lost the game in that manner,' he said.
'It's a stark reminder of what elite sport is about - the core of it is to be competitive and if you're not, it doesn't matter how good you are or how good you think you are, you're going to trip up,' Postecoglou added.
It's the 10th time that Tottenham have lost a Premier League game in which they led by two or more goals, becoming the first club to hit double figures and Brighton boss Fabian Hurzeler hailed the spirit of his side to come from behind but stressed that nothing was changed tactically at the break.
‘Honestly not much (was said at half-time). It wasn’t a tactical thing because we created chances in the first half,’ said Hurzeler.
'But without the ball, we were missing the intensity, the ruthlessness and not winning those personal duels and the ball in their half. That's how we got our self-confidence back and got in the flow and the players used the flow to create the goals.
'The half-time substitution was just to fulfil our gameplan with more energy and to be more ruthless and intense because in the end, that’s what Tottenham did in the first half to us.
'We weren’t ready for this and I’m really proud with how my team reacted because they took the responsibility, and they showed all their character and personality in the second-half. We completely deserved the win,’ added the 31-year-old.