Prop Botha has not played since injuring his elbow before Christmas, while Humphreys pulled a hamstring during training last month.
The return of both players will be a timely boost for Ulster with a string of important matches coming up for Brian McLaughlin's side.
Hooker and captain Rory Best is back in contention for a starting place after appearing as a second half replacement against Glasgow Warriors last weekend.
Young winger Conor Gaston has retained his place in the squad for the third week running after impressive performances in his first two senior appearances.
Ulster currently sit third in the Magners League after five straight wins and will be hoping to continue their rise up the table.
With their Heineken Cup quarter-final against Northampton Saints coming just nine days after the Scarlets game, McLaughlin will be hoping for an improved performance after scraping past the Dragons and Glasgow in their last two outings.
The Scarlets occupy sixth position but are just five points adrift of Ulster and with a Magners League semi-final spot up for grabs, it promises to be a closely-fought game.
ULSTER Squad (v Scarlets) - Backs (17): Ruan Pienaar, Paul Marshall, Ian Humphreys, Niall O'Connor, Ian Whitten, Nevin Spence, Paddy Wallace, Darren Cave, Mark McCrea, Andrew Trimble, Simon Danielli, Tommy Seymour, Craig Gilroy, Conor Gaston, David McIlwaine, Jamie Smith, Adam D'Arcy.
Forwards (18): Jerry Cronin, Declan Fitzpatrick, Tom Court, BJ Botha, Paddy McAllister, Bryan Young, Rory Best, Nigel Brady, Andi Kyriacou, Ryan Caldwell, Dan Tuohy, Johann Muller, Tim Barker, Pedrie Wannenburg, Thomas Anderson, Willie Faloon, Chris Henry, Robbie Diack.