Jim Mallinder felt a lack of accuracy was to blame for Saints’ failure to break down Castres’ stubborn defence on Friday night.
Northampton won 18-12 at Franklin’s Gardens thanks to three penalties apiece from Stephen Myler and Ryan Lamb.
But they couldn’t cut through Castres, who refused to wilt and eventually earned a losing bonus point through a late kick from the impressive Rory Kockott.
It leaves Saints out of the running for top spot in Heineken Cup Pool 4, with Ulster topping the table thanks to their 23-6 victory against Glasgow Warriors at Ravenhill.
But Mallinder says his men will fight to the last for a best runners-up spot when they head to Scotland to face the Warriors next weekend.
“It was a dificult game,” said the director of rugby. “Castres are a good side, they are difficult to beat and we struggled to break them down.
“But we’ll probably look back and we had a couple of opportunities which we should have been more accurate with.
“We’ll look at the 10-minute period when they went down to 14 men, which was a good opportunity for us to score some tries, but we’ll take the win and go up to Glasgow still fighting in the competition.”
Saints put their opponents under the cosh throughout but Castres defied fears about French sides’ mental frailties on the road.
And the travelling team emerged with huge credit from the clash as they denied Northampton time after time, including during Anton Peikrishvili’s sin-binning.
“We spoke about getting ahead, scoring a try and maybe Castres giving up but we never got to that,” said Mallinder, who saw his side fail to score a try at home for the third game in succession.
“They were always within seven points and always within that victory and we just need to be more accurate.”