Ange Postecoglou raged at his Nottingham Forest players after they crumbled in stoppage time to crash out of the Carabao Cup on Wednesday night.
The Premier League side appeared to be cruising towards the fourth round after establishing a 2-0 lead through Igor Jesus' brace.
The Welsh hosts mounted an extraordinary comeback that began with Cameron Burgess reducing the deficit with twenty minutes left to play.
The drama reached its peak in stoppage time when Zan Vipotnik levelled the score, seemingly forcing extra time.
Ange Postecglou was left fuming with his side after they squandered a 2-0 lead
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But in the seventh minute of added time, Burgess struck again to complete a remarkable turnaround and send the Championship side through.
Postecoglou delivered a scathing assessment of his team's mentality following the defeat. "I just got a sense the players thought it was sort of just going to happen automatically - that we were totally in control of the game," he said.
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"It was very disappointing. It was a game we were well in control of and we should have seen it out a lot better than we did,
"We certainly had the opportunities to kill the game off in the second half. I just got a sense the players thought it was sort of just going to happen automatically - that we were totally in control of the game.
"Tonight was a great opportunity for us to progress in a cup and give ourselves an opportunity to get that success. To let it slip is hugely disappointing."
Forest had multiple opportunities to put the tie beyond doubt after Jesus' double had established their dominance.
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The Australian has now lost both games as Nottingham Forest head coach
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Substitute Arnaud Kalimuendo thought he had extended the advantage when he found the net after coming off the bench, only for the effort to be ruled out for offside.
"We scored a good third goal that shouldn't have been disallowed," Postecoglou claimed, though he acknowledged his team had sufficient chances to secure victory regardless.
He continued: ""It's cup football. We should have been a lot more prepared for what was going to come and handled it a lot better than we did.
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"It's not about me. It's about the club. It's about us progressing and creating an environment where we can have real success.
"We need to understand that we are going to have to earn everything we are going to get; it's not just going to come to us.
"I just got the feeling tonight that towards the end we just thought it would naturally pan out that we would win the game. That's not how football works.
Ange Postecoglou was furious after Nottingham Forest gave up a two-goal lead to lose 3-2 to Swansea
|REUTERS
"We scored a couple of really good goals. We scored a good third goal that shouldn't have been disallowed.
"At the same time, with all that, we should have handled it better in that last half-hour. We should have stayed calm. There was no need for us to be so frenetic in our football.
The Forest boss acknowledged the educational value of the setback whilst lamenting the squandered chance to advance. "You learn a lot from everything. From my perspective, you understand some of the things we need to work on so we can get to where we want to," he said.
Despite noting some encouraging individual displays from players needing match fitness, Postecoglou insisted these positives were eclipsed by the failure to progress. "All that fades away when you realise we missed a great opportunity to progress," he concluded.