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Head coach Gary Stead is taking a short break before the Indian series.
McMillan's elevation should not be too taxing. The Black Caps washed Sri Lanka 3-0 in the one-day series, so they will start as favourites.
He can also rely on an in-form top order, in particular Ross Taylor who has been in tremendous touch in one-day cricket with six consecutive scores of 50 or more.
McMillan said Taylor's century in game three ''was right up'' there with some of his best.
Taylor created a slice of history with his fine knock of 137. He became the first New Zealand batsman to score 20 hundreds in a single format.
''It was a special innings. Obviously the significance of number 20 for a New Zealand player was huge.
''But he is in just such a rare vein of form and it set a platform for us and allowed some of the other guys like Henry Nicholls to come in and play with some freedom which was outstanding.''
Nicholls walloped an undefeated 124 from just 80 balls. It was the Canterbury left-hander's maiden ODI hundred.
McMillan wondered whether people understood just how well Taylor was batting at the moment. He is averaging 92 in one-day cricket since the start of 2018.
''Those numbers are world-class and at the highest end of world class. It has put us, not only in match winning situations, but it has won games for us.''
The Black Caps will be without some hitting power in the middle order. All-rounder Jimmy Neesham will miss tomorrow's game.
He picked up a grade-one hamstring strain in the third one-dayer in Nelson. The injury will be reassessed in a couple of days.
Central Districts all-rounder Doug Bracewell will replace Neesham in the squad.
Bracewell was part of the Black Caps one-day squad for the series against Sri Lanka but was overlooked for a starting spot.
Neesham's misfortune will provide Bracewell with an opportunity to impress the selectors before the series against India and the World Cup later this year.
While the Black Caps' batting during the ODI series was hard to fault, the same is not true of the bowling. Their attack was put under significant pressure at times.
The home team struggled to take wickets at the top of the innings and some of the death bowling was mediocre.
''Sri Lanka were a dogged side - they kept throwing punches at us. I think that is the excitement for the T20 match ... is both sides have been hungry for runs.
''It is a stadium that tends to produce close matches and lots of entertainment so I would expect just the same from Sri Lanka.''