Scarlets are currently fifth and will know on Friday night what they need to do against Northampton on Sunday.
Nigel Davies’ side will be knocked out of the top-tier tournament if Pau earn three, four or five match points when hosting Bulls.
If the South African side were to win at Stade du Hameau then Scarlets could sneak fourth place and earn a tough away trip in the Champions Cup by causing a huge upset in the East Midlands.
Any match points earned by Bulls – draw or bonuses – would leave the west Walians needing to respond in kind to ensure they drop down to the Challenge Cup.
In fact, Scarlets will not be guaranteed a slot in the Challenge Cup even if Bulls endure a pointless trip to Pau.
They would also need to avoid a hammering at Franklin’s Gardens to ensure progress on points difference.
If they lose in Northampton but still qualify for the Challenge Cup then they will be seeded 12th and travel to one of the runners-up from the second-tier pool stages.
Champions Cup, Pool Four fixtures
Pau v Bulls – Friday, 16 January (20:00 GMT)
Bristol v Bordeaux-Begles – Sunday, 18 January (13:00 GMT)
Northampton v Scarlets – Sunday, 18 January (15:15 GMT)