Van der Merwe takes the place of second-rower Marshall Sykes as Townsend switches to a 5-3 bench.
Finn Russell has been passed fit to start after concerns over ankle and injuries sustained last weekend but the match comes again too soon for tight-head Zander Fagerson.
That sees D’arcy Rae get the nod again in the No.3 jersey, with Elliot Millar Mills supplying cover off the bench.
Darcy Graham will earn his 50th cap, with the Edinburgh flier and Kyle Steyn retaining the starting berths on the flanks.
Scotland (v Argentina @ Murrayfield Stadium, on Sunday, kick-off 3.10pm GMT): B Kinghorn; D Graham, R Hutchinson, S Tuipulotu ©, K Steyn; F Russell, B White; P Schoeman, E Ashman, D Rae, S Cummings, G Gilchrist, G Brown, R Darge, J Dempsey Replacements: G Turner, N McBeth, E Millar Mills, M Fagerson, J Bayliss, J Dobie, T Jordan, D van der Merwe
