However Carter is confident Guptill, who looked bewildered on his second innings dismissal at Cape Town last week - turning a ball heading towards the leg side straight to short mid wicket - will come right.
"Martin's been searching for the ball a little in front of himself," Carter said.
"We're trying to talk to him about that technically and he's been putting in a little bit of work on that. It is something we need to monitor."
Carter had heard the idea put about of a change at the top, but made it clear he's backing Guptill to come through this challenge.
"Gup is a person who can handle it at the top and hopefully he'll do that again."
And Carter raised an interesting theory. In their last two series in India and Sri Lanka, New Zealand were poor in the first tests, but bounced back noticeably in the second - particularly at Colombo, which they won by 167 runs.
Carter didn't want to suggest New Zealand may be slow starters in series, then effectively said it another way. "It's almost like we seem not to be prepared for that intensity we did face," he said. In technical terms, Carter reckons New Zealand batsmen have the wherewithal, just not consistently enough.
"I think sometimes technically we are right up there," he said.
"For example the innings BJ Watling played (42 in 3 hours at Cape Town) was technically very good. We've got [centurymaker] Dean Brownlie, who I thought technically played well. But there's always that step up to the mental pressures."
The challenge may be slightly less relentless without the hamstrung Vernon Philander in the South African lineup. But there's general acceptance the ball will swing at St George's Park where Rory Kleinveldt, a big strong lump of a fast-medium bowler, will play his third test in the outstanding Philander's place.
"We know it will swing. It always swings here," Carter said.
New Zealand left arm swing bowler Trent Boult concurred.
"There's swing there, definitely, with the new ball but I'm not sure how much it's going to hang around like the conditions in Sri Lanka or India. It's a crucial part of it to swing that new ball and use it as effectively as possible."
And Boult gave an insight into the experience of bowling at South Africa's champion batsmen like captain Graeme Smith, Hashim Amla, Jacques Kallis and AB de Villiers.
"You hear the crowd behind them," he said. "At the same time it's something pretty exciting. I definitely live to get the big players out and it's a great challenge."
