After two years in charge of the Test team, the charismatic New Zealander has been persuaded to double down on his commitment to English cricket by taking up the vacant limited-overs post and agreeing a contract extension that sees him through to 2027.
McCullum expects Test skipper Ben Stokes to go "all in" with him on a red-ball journey, tipping the all-rounder to remain at the helm with him for the next three years, and hopes to reinvigorate Buttler's fortunes after two poor World Cup campaigns cost Australian Matthew Mott his job as head coach.
McCullum will oversee the next three global tournaments and wants to bring some fun to an environment that has become increasingly angsty in recent times.
"Yeah, he's been a little bit miserable at times," McCullum joked of Buttler, who will miss the forthcoming T20 series against Australia, during which Marcus Trescocthick is interim coach, with a calf injury.
"He's not naturally as expressive as some may be but I think he's done a great job. He's an incredibly gifted player and he's a fine leader.
"My job is to get the best out of him so that all those guys that sit in the dressing room feel like they can be 10 foot tall and bulletproof when they walk out to play. I want them to know that the skipper is going to give them that extra pat on the back and and enjoy the ride with them.
"If he was to retire today, he'd go down as probably the greatest white-ball player England's ever produced, so the opportunity for the next three or four years is just to enjoy it."
McCullum, who begins his new wide-ranging brief in the new year, was speaking on the eve of the final Test of the summer against Sri Lanka.
England have five wins from five so far and can record their first perfect season since 2004 with one more victory at the Kia Oval, starting Friday. That is testament to the work McCullum and Stokes have done since taking over in 2022, overseeing radical changes in outlook and personnel.
And it now seems likelier than ever that the pair will continue their work together for two more Ashes campaigns.
"The skipper and I haven't spoken about that actually but I'm assuming he's all in. He seems like that sort of bloke," said McCullum.
"He's been incredible and our relationship is fantastic. You never know what sort of opportunities will pop up for him which will challenge him but I know how invested he is in English cricket and how determined he is to drive this team forward. I would assume that will involve him being in charge for that period of time."
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