Two players almost certain to play in Perth are opener Ben Duckett and wicketkeeper Jamie Smith.
England have pulled both out of this week’s T20s against South Africa to allow them a break before the winter.
Duckett, in particular, looked jaded during the ODIs against South Africa that followed immediately after The Hundred, which was preceded by the gruelling five-Test series against India.
“I spoke to him and said ‘I think you need to freshen up’,” McCullum said.
“I’m in the racing game and not even Phar Lap [legendary Melbourne Cup-winning horse] can go around every week. Even he needs to go to the paddock occasionally.
“He’ll have a decent break at home, as will Jamie Smith.”
McCullum also backed 22-year-old bowler Sonny Baker, who returned the most expensive figures by an England debutant in the first ODI at Headingley on Tuesday.
He did not feature in the following two matches but is in the squad for the T20s against the Proteas and Ireland.
“He’s good – I’ve been really impressed with Sonny, to be honest,” McCullum added.
“The way, even in that game, he kept running in and kept trying different things. I said to him before the game and I reiterate it now, this is just the start for him.
“We wanted to give him a taste. We wanted to get him around the group and he’ll be a part of the squads moving forward.
“He’s got an immense amount of talent and he bowls at good, high pace and is able to swing the ball both ways.
“If we can add some craft and some touch to that, as he matures and experiences this level more often, he’s just going to keep getting better and better.”
England’s 342-run in Southampton on Sunday was the biggest victory in terms of runs in ODI history, but they were convincingly beaten in the first two matches when the series was alive.
Five of England’s players were unable to train before the series opener because it was played so close to The Hundred final – and McCullum said the schedule “isn’t ideal”.
“It is the first time we’ve ever met someone on the morning of a game,” he said.
“But that’s just how it is and we’ve got to be a bit more understanding of that and try and find ways in the future to be able to try and get up and going a little bit quicker.”
