Pienaar had been chosen ahead of Steyn for the two most-recent games in Australia against the Wallabies but de Villiers believes the later will be more effective in dealing with the booing boot of New Zealand fly-half Dan Carter.
"It's a case of horses-for-courses," explained de Villiers.
"With Dan Carter at fly-half for the All Blacks were are expecting them to kick quite a bit and Frans will be able to play a key role with his big boot".
There had been talk that both Bryan Habana and Pierre Spies would be forced to miss the match in Hamilton after de Villiers delayed naming his team in order to give the star pair the best possible chance of recovery from injury.
That decision appears to have paid off as Habana and Spies have been named in the starting XV for the Boks' final Tri Nations fixture of 2009, although there is clearly still some concern over the fitness of both players.
"The medical team has worked intensively with a few players this week and they will continue to do so between now and the game," added de Villiers.
"We always said that we expected to pick up a few injuries close to the end of the Tri-Nations and it is a case of managing the players properly".
Saturday's encounter with former Lions coach Graham Henry's men could cap a fine year for de Villiers and the Springboks.
Having already beaten the Lions earlier this summer and regained their status as the No1 side in the IRB rankings, the world champions need just a single point to claim their first Tri Nations title since 2004.
15 Frans Steyn, 14 Odwa Ndungane, 13 Jaque Fourie, 12 Jean de Villiers, 11 Bryan Habana, 10 Morné Steyn, 9 Fourie du Preez, 8 Pierre Spies, 7 Juan Smith, 6 Heinrich Brüssow, 5 Victor Matfield, 4 Bakkies Botha, 3 John Smit (captain), 2 Bismarck du Plessis, 1 Tendai Mtawarira
Replacements: 16 Chiliboy Ralepelle, 17 Jannie du Plessis, 18 Danie Rossouw, 19 Schalk Burger, 20 Ricky Januarie, 21 Adi Jacobs, 22 Ruan Pienaar
Kick-off 7.35 am GMT.