England's training regime has been a point of focus this series, with only internal fixtures and no official practice match scheduled ahead of their first-Test loss in Perth.
Heading into the crucial pink-ball Test in Brisbane, Australia had played and won three day-night matches since England's last one in 2023.
But the entire team from the loss in Perth opted to skip out on playing in a day-night tour match against the Prime Minister's XI in Canberra and head straight to Brisbane.
The decision prompted questions as to whether five days of training and no match practice was the best preparation for a clash with the day-night masters.
As 'Bazball' architect McCullum saw it, England had the opposite issue.
"I actually felt like we over-prepared to be honest," England's coach said on Channel 7 after the eight-wicket loss.
"I think sometimes when you're in the heat of the battle, as we all know, sometimes the most important thing is to feel a bit fresh."
Retired England cricketers Kevin Pietersen and Darren Gough, who played 162 Tests between them, were among those to question the claim on social media.
English Test legend Geoffrey Boycott, one of the side's most outspoken critics after the loss in Perth, was similarly scathing of the Gabba effort.
"Brisbane was a horror show," he wrote in UK newspaper The Telegraph.
"England talk the talk but can't walk the walk. With this sort of batting and bowling they couldn't win an egg cup, let alone the Ashes urn."
England will have a chance to freshen up when they head to Noosa for a getaway before the third Test match that begins in Adelaide on December 17.
Paparazzi are set to be out in force trying to catch the tourists lounging around in the sun as their Ashes hopes hang by a thread.
Aside from the rebukes back home, the team received plenty of scrutiny from the Australian press this series when three players, including captain Ben Stokes, were photographed riding e-scooters without helmets.
Stokes said the Sunshine Coast sojourn would be a chance for a much-needed physical and mental break.
"We have been here four weeks and they have been pretty full on," the allrounder said.
"It is so important when teams, when they do get an occasion or opportunity, go away as a team and put the pressures of this aside for a couple of days."
Asked for his opinion on McCullum's over-preparation comment, Stokes said England needed to prioritise training smarter, not harder.
"There is a great saying that I know: 'Are you going to train to train or train to dominate?'," Stokes said.
"There is a lot of training that you see going on where you are doing it for the sake of doing it and not achieving anything.
"On the back of the first Test match, when we desperately want something like we do, maybe you can fall into that 'train to train'. I have been guilty of that myself. But I like to train to dominate."
