Van der Merwe is one of four changes to the Scarlets backline that secured victory over Zebre a fortnight ago. Aled Thomas comes in at fullback, for Jordan Williams, whilst there's a new halfback pairing of Aled Davies and Steven Shingler with Rhodri Williams and Dan Jones moving to the replacements bench.
Kirby Myhill is the only change in the pack with Emyr Phillips suffering from a hamstring strain.
Ryan Elias and Tom Williams come on to the replacements bench.
Speaking earlier this week Scarlets head coach Wayne Pivac said: "We didn't perform well out in Zebre and then we had to contend with another week's break. In terms of improvements from the last performance we've talked about it and very much put Zebre to bed.
"This is a critical game to maintain our position in that top four group. We obviously want to stay top for as long as we can but for us top four is a new era really.
"Playing at home is a big thing for us. We've already won a home game against an Irish side this season, and performed reasonably well, and most definitely want to repeat that."
Scarlets team to take on Leinster at Parc y Scarlets, Friday 16th October, kick-off 7.35pm:
15 Aled Thomas, 14 Hadleigh Parkes, 13 Regan King, 12 Gareth Owen, 11 DTH van der Merwe, 10 Steven Shingler, 9 Aled Davies, 1 Rob Evans, 2 Kirby Myhill, 3 Peter Edwards, 4 George Earle, 5 Tom Price, 6 Aaron Shingler, 7 James Davies, 8 John Barclay ©
Replacements: Ryan Elias, Phil John, Will Taylor, Tom Phillips, Jack Condy, Rhodri Williams, Dan Jones, Tom Williams
Unavailable for selection due to injury: Morgan Allen (bicep), Will Boyde (knee/foot), Steff Evans (elbow), Steff Hughes (knee), Rhodri Jones (pec), Josh Lewis (knee), Jack Payne (ankle), Emyr Phillips (hamstring), Rory Pitman (bicep), Lewis Rawlins (shoulder), Michael Tagicakibau, (hamstring), Liam Williams (foot), Scott Williams (knee)
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