Chris Boyd apologised to the Saints supporters for his side's performance after seeing them stutter in victory against Benetton.
The black, green and gold were 10-5 down after a hugely disappointing, error-strewn first-half showing at Franklin's Gardens.
But they eventually bounced back, scoring four tries after the break to make sure of a 33-20 bonus-point Champions Cup win.
It keeps Saints' hopes of qualifying for the quarter-finals alive as they remain in contention to make the last eight as one of three best runners-up.
But they will know they must produce a much-improved performance when they go to Lyon on Saturday.
"Anybody who has an ounce of respect for the game of rugby would be horrified by that performance," said Boyd after his side's win against Benetton.
"It was a lack of respect for the occasion.
"When a team comes, is not full strength, is out of the competition and you know you've got to score four tries to stay in the competition, you talk about it 100 times during the week and the fundamentals tend to go out of the window.
"We were massively guilty of that today and our skill execution and decision making were dreadful.
"I can only apologise for our performance.
"It's the first time in my tenure that I wasn't proud of what we were producing at all."
Saints also struggled against Benetton in November, going 25-8 down before recovering to win 35-32 thanks to a late Dan Biggar penalty.
And Boyd added: "It is only mindset and what would worry me would be if there was a touch of arrogance in there. That would be massively disappointing.
"You can't be like that because Benetton deserve respect. They beat Lyon a few weeks ago so it was just poor from us.
"We'll go back and we won't dwell on the performance because there was so much rubbish in there that to find the gold nuggets in there would be almost impossible.
"Even the young guys in the dressing room are apologising to each other and stuff like that.
"Dan Biggar wasn't there, Alex Waller wasn't there, Tom Wood wasn't there - and they are guys who have a fair bit to say and keep guys on task.
"From a game management and leadership focus point of view, there were a lot of guys missing - and I think that showed.
"But it's a good chance for our young guys to reflect back on it and say 'if we get in these situations again, what do we need to do better?'"