Four days after masterminding the triumph in Frankfurt that secured him a stay of execution the Australian coach watched his team concede twice in the opening 16 minutes, a deflected shot from Elliot Anderson and a header from New Zealander Chris Wood.
Forest went on to win 2-1, lifting them back to third and leaving Spurs 16th after their 18th loss of the Premier League campaign.
"It's disappointing, it's a game we should never lose, it's another one we've let slip," Postecoglou told Sky Sports.
"Really disappointed with the goals we've conceded and the most frustrating thing is our football side I thought was outstanding.
"We couldn't dominate anymore than we did or create the chances, but that's had us all year. We've just conceded poor goals and given ourselves a mountain to climb.
"There's too many games this year we've lost in that manner and that's why we're in the position we are."
The feelgood factor last Thursday's progression to the Europa League semi-finals engendered immediately drained away with Forest's early goals, sapped by the knowledge Forest are the worst team in the EPL to go behind to.
Their Portuguese manager, Nuno Espirito Santo, was sacked by Tottenham after four months in 2021 in part because his teams tend to be defensive and reactive. Spurs fans want their team to play on the front foot, with panache and daring, which is why they instantly warmed to Postecoglou and stayed with him deep into this gruesome domestic campaign.
An eighth home defeat, however, has further diminished that bond. In their last seven league games Spurs have snatched a comeback home point against Bournemouth, beaten hapless Southampton and lost the other five. The belief that as the treatment room emptied the team would improve, has long disappeared.
Spurs did not play badly on Monday night, but Forest always let the opposition have the ball and only late on were they under real pressure.
That was when Harry Toffolo, on his first start of the season, hooked Dejan Kulusevski's header off the line and goalkeeper Mat Selz first blocked Richarlison's snapshot, then brilliantly turned aside a header from the Brazilian.
Richarlison did eventually head a Pedro Porro cross - the 48th of the night from Spurs - out of Selz's reach.
That was in the 86th minute, but in the ten minutes remaining, including added time, Selz was not tested.
"Five games to go and we know what to do. We'll see where we can end, but we've played already an amazing season and we can be proud of what we've done already until now," Forest's Belgian goalkeeper said.
"We just need to continue and give everything to end up there in the top five."
Forest fans may tire of Santo's defensive approach, fans usually do, but for now they are in dreamland. In their second season after promotion they are heading for the Champions League.Â
For Postecoglou, however, judgement will be swifter, and everything now hangs on the Europa League.
With PA.
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