Mallinder’s men shipped five tries as they were trounced 41-7 in the Champions Cup clash at Stade Pierre Antoine.
They are bottom of Pool 4 after winning one of their opening two matches, but they are only two points behind Leinster, who occupy top spot.
And Saints will now look to kick-start their stuttering Aviva Premiership campaign, with the men from Franklin’s Gardens currently sitting ninth in the standings.
They have won just twice in six Premiership matches, and Mallinder knows they must earn a victory when the Cherry and Whites come calling at the Gardens this week.
“We’ve got a massive league game this week,” the Saints boss said.
“It (the defeat at Castres) doesn’t mean we’re out of the Champions Cup.
“It’s a massive disappointment to come here and get nothing, but we won our home game (against Montpellier) so we’re still in with a chance.
“But now we’ve got to really quickly refocus and with Gloucester at home on Friday, a short turnaround, it is a very big game for us.
“It (a poor performance) happens and it’s happened too many times this season, that’s the worrying thing, but we’ve shown that as quickly as we can have a bad game, we’ve had a good game.
“That’s what we need to do on Friday.”