Christian Day feels overconfidence cost Saints in their humbling defeat at Castres in October.
And the experienced lock has warned his team they must be nothing less than full-on if they are to avenge the Stade Pierre Antoine shocker on Saturday.
Saints square up to Castres in the fifth round of games in Champions Cup Pool 4.
Jim Mallinder’s men currently sit bottom of the group with just the four points they claimed from the opening-night win against Montpellier to their name.
And Day is desperate for Saints to add to their tally this weekend.
“We’ve picked a really strong team and any time we’re playing at home in a competition such as this we’ve got to take it seriously as we can end up being embarrassed,” Day said.
“We’re going out there for the win, as we always do.
“We’ve been pretty poor in this competition and the players accept that.
“We know we need to seriously improve if we’re to contend in this competition as we have done before.
“It’s not been good enough from us in the last three times we’ve appeared and we need to try to put that right this weekend.”
On the 41-7 thrashing at Castres in October, Day said: “Castres have been very difficult to beat at home, we knew it was going to be a difficult fixture and we maybe went in a little bit overconfident after beating Montpellier the week before and we got a good hiding.
“Leinster back-to-back, you can say what you want about that first game, but we were probably in the game until the sending off, but certainly the second game wasn’t up to the standard we expect at this club.
“It certainly wasn’t good enough and we’ve put that to bed.
“We knew there was things to correct and when you come up against one of the best teams in Europe you can’t go into it half-hearted otherwise you end up getting the hiding you deserve.”
Saints have turned things around of late, winning all three Aviva Premiership matches after their 60-13 evisceration at the hands of Leinster in Dublin.
And Day said: “It’s probably easy to underestimate that (New Year’s Day) win at Gloucester. It wasn’t very pretty but it was a really challenging day, a big occasion for them, with a full house.
“The weather conditions were terrible and to turn around what was a nine-point deficit at half-time was really quite pleasing for the players.
“It was Stephen Myler’s 300th game, Mikey Haywood’s 150th and that was a big result for us.
“The ones either side (against Sale and Bristol) were pleasing, but that was pretty important to us if we want to be looking up the table rather than down it.”
Saints will now look to maintain that momentum this weekend.
And Day said: “Castres are a very difficult team to play against, to be honest.
“They’ve got a big, physical pack, a good set piece, a very dangerous backline with some individual stars in it.
“They’re very well coached, niggly and very difficult to break down. That’s something we’re going to have to look to do this Saturday.”