Hooker Nigel Brady will captain the side, as Rory Best is recovering from a thigh injury ahead of Ireland's summer tour to Canada and the USA.
After his performance in Ulster's win at Connacht last week, Isaac Boss will once again start at full-back with Clinton Schifcofske, the kicker of the match-winning penalty in Galway, taking to the bench.
Declan Fitzpatrick will start at tighthead prop, due to BJ Botha's neck injury, while Ian Whitten starts at inside centre, as Paddy Wallace recovers from a head injury.
Matt McCullough is back starting at blindside flanker, while Stephen Ferris is rested ahead of the British & Irish Lions our to South Africa.
Ulster head into their final Magners League clash in sixth position in the table, while Scarlets, in seventh, are only one point behind.
Williams said: "The Scarlets are having to play three matches in a week and that can be very demanding.
"I remember we were in a similar situation last season and by the end of the third match we were absolutely out on our feet.
"We drew with them this season over there in the Heineken Cup, a game we should have won so it would be great if we could put one over on them this time.
"This is a very young Ulster side and victory in Wales would do wonders for their self belief."
Along with Best, Botha and Wallace, Andrew Trimble (knee), Bryn Cunningham (cheek), David Pollock (hip) and departing duo Paul Steinmetz (shoulder) and Kieron Dawson (knee) will miss the game due to injury.
ULSTER (v Scarlets): Isaac Boss: Timoci Nagusa, Darren Cave, Ian Whitten, Simon Danielli; Niall O'Connor, Cillian Willis; Bryan Young, Nigel Brady (capt), Declan Fitzpatrick, Ed O'Donoghue, Ryan Caldwell, Matt McCullough, Chris Henry, Robbie Diack.
Replacements: Neil Hanna, Justin Fitzpatrick, Carlo Del Fava, Willie Faloon, Ian Humphreys, Mark McCrea, Clinton Schifcofske.