The Wellingtonian is expected to play but Latham - who was unlucky to miss out on the original squad after making his international debut against Zimbabwe earlier this month - would be a more than adequate replacement.
Rotation is near-on a banned word in New Zealand sport, but McCullum said the changes weren't about that, but a different "R" word.
"It's not rotation as such," McCullum said. "It's the realisation that you can't keep flogging certain people, especially with the volume of cricket under their belt. We think the team that we put out tomorrow is going to be good enough to win, otherwise these players wouldn't be here. So from our point of view, there's certainly no excuses."
South Africa experienced a couple of minor concerns today with Graeme Smith (arm) and Hashim Amla (head cold) being monitored, but skipper AB de Villiers expected both openers to play tomorrow.
McLean Park has been kind to New Zealand, who have won 21 of their 35 one-dayers at the ground, with 11 losses, two draws and one no result.
Tomorrow's game gets under way at 2pm.
