The 22-year-old from Inverness made four appearances as a replacement last season - his debut campaign as a professional - before coming off the bench against the Toyota Cheetahs in Bloemfontein a fortnight ago.
Nick Frisby and Brandon Thomson continue as the half-backs for a third successive match, whilst an unchanged back three sees Ruaridh Jackson start at full-back with Rory Hughes and Kyle Steyn on the wings.
Scotland U20 prop George Thornton could make his competitive Glasgow Warriors debut as a replacement on Saturday night, as the summer arrival from Wasps comes into the match day 23 for the first time.
D'arcy Rae, Tim Swinson and Huw Jones all drop to the bench after starting last weekend, whilst Chris Fusaro and Johnny Matthews continue their record of being named in every match day squad so far this season.
Speaking on his selection, Rennie said: "We're pretty blessed with our depth in midfield. Even with a few guys away at the Rugby World Cup, we've still got Huw Jones, Stafford McDowall, Nick Grigg and Kyle Steyn, who's starting on the wing.
"Nick's a really good defender and someone who's played a lot of games for us, and he runs great lines with ball in hand. We started him and Huw in Bloemfontein, Huw and Stafford last week and now we're keen to give Nick and Stafford a crack together. We expect them to make an impact.
"Our intention was to start Bruce Flockhart last weekend, but he didn't quite make it. He's a powerful man, very dominant defensively and a guy who carries well. He spent a lot of last season injured, but we think he's got massive potential and hope he shows that tomorrow.
"We were keen to get George (Thornton) back to Scotland, so we're excited to give him his opportunity tomorrow. His line-out lifting is excellent and he's made good shifts around his scrummaging. We're looking forward to seeing how he goes."