The Saints and Castres find themselves in the same group for a second successive season, Dylan Hartley's team winning both of last season's encounters.
"We know that things are somewhat out of our hands and that results need to go our way, but we know that if we win the games against Castres we can put pressure on Munster and the Scarlets and keep the Pool alive into the new year," said Mallinder.
"But winning both of those games against Castres won't be easy. We played them twice last year and in pre-season this summer and know that it won't matter which 15 players they put out they will all be physical and competitive in every area of the game.
"Stade Pierre Antoine is an intimidating place to play - it's a typical French ground which is pretty basic and where the crowd is right on top of you.
"We will need to play better that we did last year if we are to get a positive result in both of those games, but we'll be approaching them with a positive and determined frame of mind.
"We were disappointed with our first two results. Against Munster we played well but did not quite hold out for the result and against the Scarlets we put in a poor performance. However we still have two points which could be crucial as the group stages go on, and we showed against Saracens that we have the quality to get results in the games we've got coming up."