Alan Dickens says there was a lot of 'disappointment' in the Saints dressing room last Saturday - despite the semi-final success against Newcastle Falcons.
The defence coach believes his players felt they had let themselves down during a sluggish start to the game that saw Falcons fly into a 21-0 lead.
It wasn't until Saints went 28-7 down that the comeback really started, with the black, green and gold eventually racking up nine tries in a 59-33 success.
It set up a Premiership Rugby Cup clash with Saracens at Franklin's Gardens on March 17.
But despite that, the Saints players were far from overjoyed.
"There is a lot of confidence that can come from it but a work-on for us is how we started the game last weekend," Dickens said.
"With and without the ball we weren't good enough for the first 20 minutes.
"From my point of view as a defence coach, we were soft last weekend and we allowed three or four easy tries in that first half.
"The pleasing thing was that in the second half we tightened up and controlled the game.
"The lads are honest professionals and there was almost a sense of disappointment in the changing room after the game last weekend.
"A lot of supporters will read that and think 'what does he mean by that?', but it was their personal pride of that first 20 minutes not being good enough on both sides of the ball.
"We leaked too many points and mentally we weren't there at quarter past 12 last Saturday.
"It's something we've talked about and something we've got to improve on this week."
Ninth-placed Saints will be back in Gallagher Premiership action this Saturday as fifth-placed Sale Sharks come calling at the Gardens on Saturday.
And Dickens said: "Sale are a team that have showed you can put a run together in this league. It's that tight that they've gone from being bottom when we played them away (in November) up to fifth now.
"They're a team that have been in form and they're certainly a team we're not going to take lightly.
"They've got Faf de Klerk at scrum-half and he's the heartbeat of their team and their back three are dangerous with people like (Denny) Solomona and Chris Ashton, who we know from his time here in terms of his ability to score tries.
"They're a team who have got threats across the board, not just in the backs.
"People like Rob Webber offer themselves up all game to carry, Jonno Ross in the back row and the Curry brothers.
"We certainly will not be taking them lightly."