The clubs are level on six points apiece at the top of Pool 6 - with the Saints facing a tricky trip to Belfast to take on Ulster six days later while Biarritz will have home advantage against Cardiff.
'I don't think I've faced a bigger challenge in my coaching career,' said the former All Blacks coach. 'We know that if we win we will go top of our Pool and be in pole position to go through to the quarter-finals. But we know how dangerous Biarritz are from our first meeting with them and it is going to be a huge challenge for us.
'We are taking all three competitions we are involved in very seriously and winning this weekend means everything to us. It is backs to the wall time because of the injuries we are having to cope with.
'I don't think Ben Cohen will be fit to play and it looks as though we'll also be without Budge Pountney, who was injured against London Irish. We have had a lot of injuries to key players recently and we've been asking a lot of our squad.
'The young players have been stepping up and showing a lot of heart. We've lost two thrilling games at home and that shows that teams are rising to the occasion of playing against us at Franklin's Gardens.
'Our supporters have been fantastic all season. You have to earn support like that by playing well week in, week out and we know we owe them a big performance against Biarritz.'