Head coach Joe Schmidt has made five changes from the team which won the first Test 26-20 in Cape Town, with Tadhg Furlong, Rhys Ruddock, Stuart Olding and Craig Gilroy also handed starts.
Powerful lock Iain Henderson will move to the back row to make space for Roux's extra bulk, which Quinlan believes will help their game plan as they seek to go 2-0 up in the three-match series.
And Quinlan also backed Ulster's Olding to make an impact after being preferred to club teammate Luke Marshall at inside centre.
"They are all good players who have come in and replaced the guys that started last week in that brilliant win in Cape Town," he told Sky Sports.
"The big talking points are Quinn Roux and Stuart Olding - they're two guys who maybe weren't expected to start.
"Joe Schmidt is a fan of Quinn Roux who hasn't played a lot of rugby this year but he's said that he brings a lot of value in the set-piece and physicality that they're going to expect from the Springboks this week.
"Stuart Olding is a very talented player who has had a good season with Ulster - again Joe Schmidt compared him to a former great in Gordon D'Arcy so I don't think the changes are going to affect the team, in fact they might boost the team.
"The players are fresher, it's going to be physical again like it was last week in Cape Town.
"It's going to be very difficult - they made history last Saturday in Cape Town and it was an incredible performance considering they had 14 men for most of the game."
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