|European Champions Cup|
|Venue: The Recreation Ground Date: Friday, 12 January Kick-off: 19:45 GMT|
|Coverage: Live commentary on BBC Radio Wales, BBC Radio 5 live sports extra, online and the BBC Sport app, plus live score updates.|
Bath welcome back Sam Underhill for the vital Champions Cup Pool 5 game - his first match since being concussed while playing for England in November.
Matt Banahan - their longest-serving player, who has signed for Gloucester next season - and 6ft 9in Wales second row Luke Charteris also return.
Leigh Halfpenny and Johnny Mcnicholl failed fitness tests for Scarlets who are third, a point behind leaders Bath.
But Wales backs Gareth Davies and Scott Williams both return.
Halfpenny's chest muscle injury means Rhys Patchell - initially named at fly-half for the game - switches to full-back.
With two rounds remaining, both sides need to win on Friday night in the West Country.
Bath are on 13 points, one clear of second-placed Toulon and third-placed Scarlets, who host the French three-time European champions in the final round next Saturday.
Bath, meanwhile, end the group stage with a trip to bottom side Benetton Treviso on Saturday.
Bath won 18-13 at Scarlets back in October and Scarlets boss Wayne Pivac is determined to reverse that result.
"This week is about trying to get our attacking game going again," he said.
"We just have to look back to when we played them last time - they were more disciplined and more clinical than we were and played the conditions better than we did.
"We're really looking forward to the occasion. It's a must win game for both sides. We both know that. It's like a knock-out game in itself."
Bath: Watson; Banahan, Joseph, Tapuai, A Brew; Priestland, Cook; Obano, Dunn, Thomas; Stooke, Charteris, Garvey (capt), Underhill, Mercer.
Replacements: Van Vuuren, Noguera, Lahiff, Phillips, Grant, Fotuali'i, Burns, Wilson.
Scarlets: Patchell; Asquith, Parkes, Williams, Prydie; D Jones, G Davies; Evans, Owens (capt), Lee, Beirne, Bulbring, Shingler, J Davies, Barclay
Replacements: Elias, W Jones, Kruger, Rawlins, Macleod, ADavies, Hughes, Boyde
Source: BBC Rugby Union News