The visitors will be without suspended players Chris Ashton and Phil Dowson at Stade Pierre Antoine (1.30pm), while injury sidelines number eight Roger Wilson from a back row that has an unfamiliar look of Tom Wood, Ben Nutley and Samu Manoa.
The Saints head to the south of France lying third in Pool One after winning bonus points against Munster and the Scarlets.
But while Mallinder acknowledged that it hadn't been the best of starts for his team, there was plenty of desire among the players to stay in the hunt for a place in the knockout stages.
"We were disappointed to lose our first two games in the Heineken Cup but it doesn't stop our desire to win this game and the rest of the games," he said.
"We know we can still qualify. It's still in our hands and we're going to go to Castres very determined.
"All of the attention is probably going to be on the other two teams at the top of the pool and probably not going to be on us. This means that we can concentrate on our own job in hand which is winning in Castres.
"We're in a good frame of mind, good spirits and positive that we can do that."
A number of suspensions and injuries have meant that Mallinder has introduced several changes to the team which started at Welford Road last Saturday, but instead of being downbeat he said that it was a great opportunity for the players who are coming in to show what they can do.
"We're disappointed that we've lost a few players, but the lads who are coming in are capable of getting a positive result on Saturday," he added.
"We've spoken all season about the strength in depth we have here and we've got confidence in all the players to play."
Castres have recalled Romain Teulet to the full-back jersey for their clash with fellow strugglers Northampton at Stade Pierre Antoine on Saturday.
Only international winger Max Evans retains his place in the home backline following their defeat to Munster in Round Two, with Romain Martial, Thomas Sanchou, Seremaia Bai, Rémi Tales and Rory Kockott all making Heineken Cup starts.
There is also a new flank partnership as Steve Malonga and Ibrahim Diarra replace Yannick Caballero and Jannie Bornman while Rodrigo Capo Ortega is in at second-row for Iosefa Tekori and Luc Ducalcon, Marc-Antoine Rallier and Michael Coetzee form a new-look front-row trio.
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