Forest surged into a 2-0 lead within 16 minutes through Elliot Anderson and Chris Wood, capitalising on Spurs' sluggish start. Despite a late response from Richarlison in the 87th minute, the hosts could not salvage a point.
Postecoglou, reflecting on the loss, said: "It's a game we should never have lost. The goals we conceded were poor, and we gave ourselves a mountain to climb. Our football was outstanding at times, but you can't give teams a two-goal head start."
He highlighted a recurring issue, noting: "We've seen this too often this season, lacking concentration for the full game. It's cost us in matches like this and explains why we are where we are in the table."
The defeat, Spurs' second in their last five league games, was compounded by Postecoglou's admission that his team selection was influenced by their upcoming Europa League semifinal first leg against Bodo/Glimt.
Key players Micky van de Ven, Cristian Romero, Dejan Kulusevski, and Wilson Odobert were limited to 45-minute appearances to manage their fitness.
"It was pre-planned," Postecoglou explained. "We need everyone healthy for Thursday's big game. It's a balancing act, but we missed a chance to keep momentum tonight."
Looking ahead to a daunting trip to Liverpool this weekend, Postecoglou remained defiant: "If we play like we did for most of this game but maintain our focus, we can give them a real test.
"We just need to fix a few things." Tottenham have struggled for consistency this season, sitting mid-table, while Liverpool are pushing for the title.
Meanwhile, Richarlison, who scored Spurs' only goal, posted on X: "Gutted with the result, but we keep fighting. On to the next one." The club's official account confirmed no fresh injuries, with Van de Ven and Romero expected to be fully fit for the Bodo/Glimt clash.
Tottenham's defeat underscores their need for sharper focus as they juggle domestic and European ambitions. With Liverpool looming, Postecoglou's side face a critical moment to prove their mettle.
Marcus Rashford, the Manchester United forward, has reportedly set his sights on a blockbuster transfer to Barcelona in the summer of 2025, despite his impressive form during a loan spell at Aston Villa.
Lyle Foster's Burnley have secured their promotion back to the Premier League.
Arsenal are reportedly considering Crystal Palace's Adam Wharton as a replacement for Thomas Partey, whose contract is nearing its end, former Manchester United midfielder Owen Hargreaves has said.
Burnley and Leeds United have been promoted back to the Premier League after both teams secured victories in the English Championship on Monday.
