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However it's hard to think of one worse, as New Zealand's series came to a predictable and thumping 2-0 series defeat against the world's best team.
Having been dismissed for 211, 35 minutes before lunch on the fourth day at St George's Park, they lost by an innings and 193 runs - to follow the innings and 27 runs at Cape Town a week earlier.
"No, I wouldn't go to that extreme," McCullum, in his first series as test captain, said.
"Obviously it's incredibly disappointing and hurts a lot to suffer a 2-0 loss and in the fashion we have.
"But the acknowledgement of the different class between the teams is important for perspective. Yes we weren't good enough and didn't front up when we needed to, but we were also placed under tremendous pressure by a team at the very top of their game, and that should provide some learning opportunities for us.
'Yes, it should hurt but we've got a pretty good blueprint of how the best team in the world goes about its cricket."
Twelve sessions were unrequired over the two tests for South Africa to take their run of successive series wins to five - home to Sri Lanka, away to New Zealand, England and Australia and now this.
In that time they've lost just one test, against the Sri Lankans in Durban in the Boxing Day test last year.
McCullum knows there has been, and will continue to be strong criticism of New Zealand's performances in the tests. His team "certainly won't shy away from that", he added.
"But also a lot of people will support us and understand that this team, in its life cycle, is greatly different from the current South African team and hope to see some improvement in what is going to be a big series coming up (home to England)."
South African captain Graeme Smith believed the final session on day two, when his team reduced New Zealand to 47 for six in reply to the hosts' 525 for eight declared, decided the match.
"We really got things together and once you create that intensity and pressure, the snowball effect (comes in). It rubs off and that evening was where the game opened up for us," he said.
His team had bowled expertly in the first innings of both tests, rolling New Zealand for 45 in 19.2 overs on the opening morning of the series at Cape Town and then 121 in Port Elizabeth.
"In both first innings we bowled really well and were able to create a lot."
Smith believed the series outcome was a fair reflection of the teams.
"If you play two tests and win both of them by an innings, then that's fair to say. I'd like to think we outplayed them comprehensively in the series.
"We've been really professional, bar one or two catches in Cape Town. We've played a high standard of test cricket," Smith said.
New Zealand began the day at 157 for four, with both Dean Brownlie and BJ Watling set and in their 40s.
Once Brownlie departed at 53 the end came swiftly.
The last five wickets fell for just eight runs in the space of 30 balls from Dale Steyn and Morne Morkel.
World No 1 Steyn, in particular, was unplayable late in the innings, his delivery to remove the doughty Watling being a gem.
He finished with overall figures of eight for 65 to win the man of the match award.
"Having guys that keep coming all day and have the ability to create pressure and keep bowling with intensity and pace and get the ball to move is crucial," Smith said.
''When things are going well and bowlers are bowling quick then it's great. It's a relentless thing that people can back each other up."
McCullum, who made 82 runs in his four innings at the top of the order, believed it to be the toughest batting experience he'd faced.
''I've never been challenged like that at the crease consistently from a group of bowlers. It was as hard as you get in international cricket. They just give you no scoring opportunities," he said.
Brownlie topped the New Zealand batting averages with 43; Watling was one run lower. Forget the rest.
McCullum said there will have to be ''nipping and tucking" before the ANZ international series against England. However he felt the core of the team should be retained to implement what they learnt in the two tests.
All positions would be up for discussion.
There promises to be some hard talk, with former captain Ross Taylor tipped to return, issues at the top of the order - and McCullum was non-committal on whether he would stay opening - and the mix of the middle group on the table.
For now, New Zealand must regroup smartly for the three-game ODI series, starting in Paarl on Saturday night (NZT).
- David Leggat