With London Irish out to revenge their defeat at home in Reading against the Scarlets, both teams have admitted it is a 'massive match' - knowing that a result keeps their hopes in the Europe alive.
Scarlets Head Coach Nigel Davies said the Parc y Scarlets home crowd will have plenty to look forward to. "It should be a good game, both sides traditionally look to play an expansive brand of rugby. I've got a lot of admiration for what they are doing at London Irish, they are a quality outfit and with both sides knowing it is a must win match - it should make for a great game of rugby, just like the first game was," he said.
The Scarlets welcome back experienced prop Iestyn Thomas, flanker Josh Turnbull and Morgan Stoddart.
The Scarlets are determined to put their back-to-back form against Leinster right, with Davies saying it is now the 'key game' in the European Cup with "a lot to play for and a lot to prove".
He said: "This is a key game in our group now. It's very important to both teams and realistically we are still in there with a shout of finishing in second spot in this particular group if we win on Sunday.
"If we can do that in this group, that will be a big achievement for us, particularly where we've come from over this past two or three years."
And Davies backed his young Scarlets fly-half, Priestland, saying he has developed well this season: "I am really pleased with Rhys, I think his overall game is very good, and he has developed really well this year. He has a bit more of a mental hardness to his game now and affects games very positively. I felt that in our win against the Dragons on New Years Eve, he had a significant impact on the game."
London Irish second row Bob Casey who will lead the Exiles out after back to back wins against Brive. He admits his side are aware of the challenge the Scarlets present.
"We slipped up badly at home when the Scarlets punished us but we will be doing our homework on them and make sure we are 100 per cent prepared for what we know will be a huge challenge in West Wales," said Casey.
"The Scarlets have a decent scrum, their line-out is going well and they have gain breakers behind the scrum and guys who can run in tries from all over the pitch.
"Any team that can do that is a threat and in David Lyons they have one of the best No 8s in the competition, an extremely powerful player with a great rugby brain."
Scarlets 15 Daniel Evans, 14 Morgan Stoddart, 13 Sean Lamont, 12 Jonathan Davies, 11 Andy Fenby, 10 Rhys Priestland, 9 Martin Roberts, 1 Iestyn Thomas, 2 Ken Owens, 3 Deacon Manu, 4 Lou Reed, 5 Dominic Day, 6 Rob McCusker, 7 Josh Turnbull, 8 David Lyons (captain).
Replacements: 16 Emyr Phillips, 17 Phil John, 18 Jamie Corsi, 19 Damian Welch, 20 Johnathan Edwards, 21 Lee Williams, 22 Dan Newton 23 Gareth Maule.