Having sustained injuries in last weekend's 30-6 derby win over Connacht, Simon Danielli and Dan Tuohy have joined Ulster's injury list.
Rory Best, David Pollock, Bryn Cunningham, Matt McCullough and Cillian Willis are also not currently available for selection due to various injuries.
But Ireland and Lions flanker Stephen Ferris is in contention make the matchday squad. He sustained a back injury in training last week, which meant he was a late withdrawal for the Connacht game. A decision on his fitness will be made on Thursday.
Commenting on the squad's development to date this season, head coach Brian McLaughlin said: "We are making progress and are starting to play good, strong rugby, but we have a bit of distance to go yet.
"There are still improvements to be made and skills to be perfected, but we will continue to work and improve and fight our corner every week.
"The players are working extremely hard and buying into the ideas that I and the other coaches - Jeremy Davidson, Peter Sharp and Neil Doak - are bringing to them.
"We are pleased and positive about how things have gone to date."
McLaughlin's men are fourth in the Magners League table ahead of their televised clash with the second-from-bottom Scarlets.
The province's team and replacements will be confirmed on Thursday afternoon.
ULSTER Squad (v Scarlets): Backs (12) - Timoci Nagusa, Darren Cave, Andrew Trimble, Ian Whitten, Paul Marshall, Isaac Boss, Ian Humphreys, Paddy Wallace, Clinton Schifcofske, Ian Porter, Niall O'Connor, Mark McCrea.
Forwards (14) - Nigel Brady, BJ Botha, Bryan Young, Ed O'Donoghue, Andy Kyriacou, Chris Henry, Stephen Ferris, Willie Faloon, Thomas Anderson, Neil McComb, Tom Court, Declan Fitzpatrick, Ryan Caldwell, Tamaiti Horua.