Ireland's Rory Best and Stephen Ferris are being rested as part of a player management programme implemented post-World Cup.
Court, Wallace and Trimble received limited game-time as Ireland exited at the quarter-final stage and returned to training with their province this week.
South Africa were knocked out over the same weekend, with Pienaar and Muller playing periphery roles towards the end of their campaign.
Young centre Luke Marshall has been ruled out after fracturing his jaw in a club game two weeks ago, while prop Jerry Cronin has sustained a neck injury and is due to get a scan.
The international players return at the perfect time for Ulster, who have tasted defeat in their previous three league games. The leadership of Muller will be vital in improving Ulster's recent discipline and set piece difficulties.
ULSTER Squad (v Scarlets) - Backs (15): Paul Marshall, Ruan Pienaar, Ian Porter, Ian Humphreys, Paddy Jackson, Paddy Wallace, Nevin Spence, Ian Whitten, Darren Cave, Craig Gilroy, Simon Danielli, Andrew Trimble, Conor Gaston, Peter Nelson, Adam D'Arcy.
Forwards (17): Paddy McAllister, Callum Black, Tom Court, Adam Macklin, Declan Fitzpatrick, Andi Kyriacou, Nigel Brady, Lewis Stevenson, Johann Muller, Dan Tuohy, Tim Barker, Neil McComb, Mike McComish, Pedrie Wannenburg, Willie Faloon, Chris Henry, Robbie Diack.