With his squad already severely depleted, Davies has seen prop Iestyn Thomas forced out with a neck problem. His place will be taken by Phil John with young Aled Hopkins drafted in as cover from the ranks of Llanelli's Principality Premiership side.
Darren Daniel is one of two other changes. He replaces Ceiron Thomas on the right wing to make his 50th appearance for the Scarlets. Thomas drops down to the bench while Rhys Priestland replaces the injured Morgan Stoddart at full-back. Although a fly-half, Priestland is not entirely unfamiliar with the No15 shirt as he has played semi-pro rugby in that position.
After a lengthy spell on the sidelines, Scotland second row Scott MacLeod makes a welcome return to regional action and is named among the replacements.
Scarlets: R Priestland; D Daniel, R King, J Davies, M Jones; S Jones, M Roberts; P John, M Rees, D Manu, V Cooper, D Day, D Jones, S Easterby, D Lyons.
Replacements: K Owens, A Hopkins, S MacLeod, J Edwards, G Cattle, C Thomas, L Williams
Ulster Rugby: B Cunningham; A Trimble, D Cave, P Wallace, S Danielli; I Humphreys, I Boss; J Fitzpatrick, R Best [capt], B Botha, E O'Donoghue, R Caldwell, S Ferris, D Pollock, R Diack
Replacements: N Brady, T Court, M McCullough, K Dawson, P Marshall, P Steinmetz, C Schifcofske.
Referee: Romain Poite (France)