The Scarlets pipped the powerful French club 19-15 at Stradey Park last weekend, and director of rugby Gareth Jenkins knows how tough the return leg will be.
"There will be a reversal of roles this weekend," Jenkins said. "The expectation and the pressure is on them - the whole of Agen will be expecting a home result.
"Saturday is a great opportunity for us - this weekend is critical but there is a confidence in our team. If we can go there and win on Saturday it will put us in a great position - but it's a huge ask."
The Scarlets pack is pleased with the way the set pieces went last Friday night, particularly the scrum, but prop John Davies says Agen captain Jean-Jacques Crenca, returning from suspension, will make a big difference.
"He is a top international loosehead and he'll be itching to get back in and impose expertise and authority," Davies said. "It's a difficult challenge."
Llanelli Scarlets: B Davies; G Evans, M Taylor, M Watkins, S Finau; S Jones, 9 D Peel; I Thomas, R McBryde, J Davies, V Cooper (capt), C Wyatt, D Jones, S Easterby, S Quinnell.
Replacements: P John, A Gravelle, A N Other, D Hodges, M Phillips, G Bowen, M Jones.