"I don't think I've got enough superlatives to describe that innings."
McMillan reckoned the skipper's 202 against Pakistan at Sharjah last month was "pretty special". Yesterday exceeded that.
"The man is an x-factor player, has the ability to dominate and dismantle bowlers very quickly and change the tempo and way an innings is heading."
There was a happy spinoff, too, in the bracing 85 by Jimmy Neesham, a player short of runs from the last tour to the United Arab Emirates.
McMillan felt McCullum's presence helped the lefthand allrounder, who hit centuries in his first two tests but fell away against Pakistan.
"It was a really important knock for him, just to see the confidence seeping back into his system. Batting with Brendon almost dragged him along and gave him confidence to be more expressive," McMillan said.
And he revealed McCullum was relatively relaxed about missing out on another double century.
"I think he felt that's the way it goes sometimes. He rolls with it. Live by the sword, die by the sword. But any player if you get that close to a double hundred, there's always maybe a little bit of disappointment."
So how to account for McCullum's dazzling run of form, given that his modus operandi has changed little down the years? McMillan admitted he has no simple answer.
"Maybe it's the captaincy, if you look at the number since he became captain. But as a player there's a time in your career when everything clicks and comes into place. Perhaps over the last 12-14 months that's where Brendon is in terms of his career.
"It might just be his time but he's certainly making hay and batting beautifully."
McMillan also raised the idea that a tighter defence and a willingness to defend and respect the good delivery may also have played a significant part in his rare form since the start of the year.
"It might sound crazy when he's belting the ball around the park, but I think he's tightened up in certain areas of his game. He's got all the shots, but his option-taking, based around sound defence, has taken his game to another level."