Heineken Cup winner Dom Waldouck believes Northampton Saints are still right in the Pool 4 mix for quarter-final qualification but need two 'big results' to keep their Heineken Cup hopes alive.
Ulster Rugby are in pole position on 15 points at the top of the group with Castres Olympique on 12 points and the 2000 European champions two further points adrift.
A George Pisi try helped the Saints leave Stade Ernest Wallon with a bonus point from Castres' 21-16 win in Round 2 but Friday night's showdown with the club riding high in fourth place in the Top 14 is a must win contest for the Saints.
"It's a huge competition for us and we're still alive," said former London Wasps centre Waldouck, a tournament winner in 2007 when Wasps stunned Leicester Tigers with a 25-9 victory.
"We need two big results in the next two weekends and hopefully we can take some momentum into the Heineken Cup and into the rest of the season."
The momentum comes from a 30-19 win at Exeter Chiefs when Saints hit back at their critics but Waldouck says that only spurred his side on.
"We have had a lot of criticism in the media but we had only lost by a point the week before to the top team in the Premiership - Saracens - and we'd come under a lot of criticism so it was important to go out and prove that we believe in ourselves.
"There aren't many trophies won in January so we can make a push at the end of the season."
The Saints complete their Pool matches for the season with a trip to Glasgow Warriors eight days later.