Stephen Myler has urged Northampton to "keep the momentum going" when they bid for an unlikely Heineken Cup quarter-final place.
Pool One losses earlier this season against Friday night's Franklin's Gardens visitors Castres and group leaders Leinster have left Saints facing a group stage exit.
Their only hope is to beat Castres with a bonus point, and then hope the three-time European champions collect nothing from a Dublin appointment with the already-eliminated Ospreys.
Northampton, currently second in the Aviva Premiership, can expect the probable consolation of a spot in the second-tier Amlin Challenge Cup last-eight if they defeat Castres, but Leinster emphatically hold pole position.
"Castres are the French champions and we know it's going to be a tough test," Saints fly-half Stephen Myler said.
"Heineken Cup qualification is not in our hands, but we can look after what we can control, and that is the Castres game, then we will see where that takes us.
"The important thing is that we keep the momentum going which we've built over the festive period, keep pushing forward and aiming to improve."
Saints rugby director Jim Mallinder has made two changes from the side that saw off the Ospreys 29-17 in Swansea last weekend, calling up full-back Fa'atoina Autagavaia and scrum-half Lee Dickson.
The Ospreys might be out, but they parade six British and Irish Lions in their pack against Leinster, including back-row forward Ryan Jones, who has recovered from a hamstring injury.
"We may be out of contention, but we aim to be competitive in every game we play," Ospreys head coach Steve Tandy said.
"Looking back at our fixtures throughout this competition, we have been in every game but key moments haven't gone our way and we've paid the price. Those are the margins at this level.
"We know that we need to be more clinical. We have to take the opportunities when they present themselves or we will find ourselves thinking about the 'what ifs'.
"We will be looking to the senior players in the squad, those who go away with Wales after the weekend, to put in big performances before they go into camp for two months, setting the standard for the younger boys."
Leinster show four changes up-front from the side that won in Castres last Sunday, while there is an earlier-than-expected return on the bench for hooker Richardt Strauss.
The Ireland international was diagnosed with a heart condition last October and was initially ruled out for the rest of this season, but his progress has been such that Leinster's medical team have now cleared him to play.