Aidy Boothroyd said a complacency among his Cobblers side was to blame for their emphatic defeat at Chesterfield.
Town went into the game on the back of a three-game winning streak that was shredded by a Spireites side that scored three goals in seven first-half minutes to end the game as a contest.
Former Northampton striker Marc Richards helped himself to a break with Jay O’Shea also on target in an opening half an hour in which the home side were rampant.
Boothroyd’s men completely failed to live up to their recent billing as a side capable of challenging for promotion, although they remain just a point outside the npower League Two play-off places.
The manager felt the team went into the game at Chesterfield with the belief that they were better than their recent reputation would suggest.
“This always happens to a team that is in a development stage and I’ve had it on a number of other occasions,” he said.
“What it is, when you go on a little run and people start to talk about you, you have the opportunity to build a reputation and that reputation can sometimes be better than what you actually are.
“What happens is the teams you play against up their game because they are frightened of you.
“I read all the press about Chesterfield before the game and they were all talking about how they had to be up for it and how they had to be at it because Northampton are a good side.
“I have guarded against it and I have spoken about expectation. It is very disappointing because we never started playing for 30 minutes and you cannot do that against a team that is up for it.
“We were complacent and we thought we were better than we were. Because of that we got a hiding. When you get a hiding you have to take it and I do take it but we know we’re better than that.”