Saints have lost their opening three fixtures in Pool 1, leaving last year's beaten finalists with little chance of reaching the knockout stages.
They did manage to pick up a losing bonus point away to Castres, giving director of rugby Jim Mallinder something positive to build on in training ahead of the return fixture at Franklin's Gardens.
"We had good attacking intent and scored some good tries last week," he said.
"But we didn't keep hold of the ball well enough and conceded too many penalties. This is something we're going to have to put right this weekend, along with our defence.
"We still want to do as well as we can, and if we can get into the Amlin Challenge Cup then we'll take it without a doubt as it's another opportunity. But to be in with a chance of that we've got to start winning in the Heineken Cup, starting this weekend.
"The players want to prove a point. There are no 'nothing' games at this level and there has been a good response in training this week.
"You're representing the club every time you pull on the shirt and I expect that the team will come out on Sunday fully motivated for what will be a tough challenge against a Castres outfit that showed their quality last Saturday."
Mallinder has gone for Jon Clarke and Tom May as his centre pairing while Alex Waller comes in to replace Soane Tonga'uiha, who drops to the bench, in the front row.
Meanwhile, Castres have chosen not to play Romain Teulet or Chris Masoe in the away game - the duo are replaced by Florian Denos and Jannie Bornman at full-back and number eight respectively.
Max Evans is not included in the squad while Rory Kockott, Seremaia Bai and France prop Luc Ducalcon make the replacements bench.
Northampton: 15 Ben Foden, 14 Vasily Artemyev, 13 Jon Clarke, 12 James Downey, 11 Jamie Elliott, 10 Stephen Myler, 9 Martin Roberts, 8 Samu Manoa, 7 Ben Nutley, 6 Tom Wood, 5 Mark Sorenson, 4 Courtney Lawes, 3 Paul Doran-Jones, 2 Dylan Hartley, 1 Alex Waller.
Replacements: 16 Mikey Haywood, 17 Soane Tonga'uiha, 18 Tom Mercey, 19 James Craig, 20 James Ingle, 21 Lee Dickson, 22 Greig Tonks, 23 Scott Armstrong.
Castres: 15 Florian Denos, 14 Matthieu Nicolas, 13 Paul Bonnefond, 12 Pierre-Manuel Garcia, 11 Pierre-Gilles Lakafia, 10 Pierre Bernard, 9 Thierry Lacrampe, 8 Jannie Bornman, 7 Steve Malonga, 6 Pierre Roussel, 5 Scott Murray, 4 Matthias Rolland, 3 Rossouw Kruger, 2 Mathieu Bonello, 1 Anton Peikrishvili.
Replacements: 16 Marc-Antoine Rallier, 17 Michaël Coetzee, 18 Luc Ducalcon, 19 Rodrigo Capo Ortega, 20 Ibrahim Diarra, 21 Rory Kockott, 22 Seremaia Bai, 23 Vincent Inigo.
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