The former Neath and Swansea outside-half, who arrived in the summer from Pau via the Celtic Warriors, will take over from the injured Gareth Bowen for the Celtic League clash. It will be a rare start for Thomas, 29, who struggled to gain any regular rugby in France last season amid a string of injuries after leaving Swansea where he was more used to being on the other side of this local clash.
But he now gets his first chance to become the long-term successor to Stephen Jones after young rival Bowen was ruled out with an ankle injury. 'I always looked forward to the Llanelli fixture, the rivalry that has built up between the supporters has always added to the game, It was always a great occasion to be involved in,' he said.
Thomas is one of four changes, one positional, to the side that opened the defence of their Celtic crown with a 26-15 victory at The Borders last week. Barry Davies returns at full-back so Garan Evans moves across to the right wing in place of Aisea Havili while Mark Taylor comes back into the side for Tal Selley.
Up front, Vernon Cooper takes Bryn Griffiths' place in the only change to the pack. The Ospreys have yet to name their team but will be without Shane Williams and Elvis Seveali'i who both impressed during their victory over Munster. Williams suffered a badly cut hand while Samoan centre Seveali'i is out for six weeks with a torn calf muscle.
Meanwhile, Ospreys coahc Lyn Jones has had to select his side without a host of his first choice picks.
Huw Bennett is still receiving treatment on his groin injury. Luke Tait is being monitored re his neck injury, and we are hopeful he will be available for selection within the next two weeks.
Elvis Seveali'i is undergoing intensive rehab on a (soft tissue) calf injury, but things are progressing well and we hope he will be fit in 3-4 weeks.The injury to Shane Williams' hand is still being assessed
Shaun Connor is recovering well from a haematoma of the right leg, but hopes to be in contention for selection for The Blues fixture.
Scarlets: B Davies; G Evans, M Watkins, M Taylor, S Finau; A Thomas, M Phillips; I Thomas, M Rees, J Thiel, V Cooper, C Wyatt, S Easterby (capt), S Quinnell, G Thomas.
Replacements: J Davies, C Hawkins, A Jones, D Jones, R Walters, T Selley, A Havili.
Ospreys: 15 Gavin Henson, 14 Paul Mackey, 13 Sonny Parker, 12 David Bishop, 11 Stefan Terblanche, 10 Matthew Jones, 9 Jason Spice, 8 Ryan Jones, 7 Richard Pugh, 6 Jonathan Thomas,
5 Brent Cockbain, 4 Andy Newman, 3 Andrew Millward, 2 Barry Williams (Captain), 1 Duncan Jones.
Replacements: To be announced