Peterborough manager Darren Ferguson has said the heavy defeat to Northampton has "taken the air out" of his team ahead of a crucial week that includes the EFL Trophy Final.
The 4-0 defeat, described as "abject" by Ferguson, ended a run of one defeat in nine games in all competitions, that had lifted Posh clear of the League One relegation zone.
Peterborough now face League One leaders Birmingham City twice in the space of six days, first on Tuesday in the league, and then at Wembley as they defend their EFL Trophy crown on Sunday.
"When you've got a lot of momentum, you need to carry that on going onto a big week," Ferguson told BBC Radio Cambridgeshire.
"I have never seen in my time, and I've managed hundreds of games here, seen the stadium that empty [towards the end of the game against Northampton].
"This was a derby, it was a big game for the fans, and we let them down massively."
'Act of ill-discipline'
Peterborough are on 48 points, nine clear of the relegation zone with six games remaining, but were dominated by rivals Northampton, particularly after Tayo Edun's first-half red card.
"It's a total act of ill-discipline", Ferguson said. "It's not what we're about.
"I'm told he punched him in the face. It's something we can't do."
Despite a big date at Wembley on the horizon, Ferguson said he would be solely focused on Tuesday's game.
"The big week was today and Tuesday, not next Sunday. I keep saying it, I have no interest in [the final]. We're only on 48 points," he added.
