Harry Brook is the clear favourite to step up but has only recently become vice-captain and, unlike Morgan or Buttler before him, is an all-format regular. To play in all three formats for England while captaining in two would be unprecedented in the modern era, juggling two months at the IPL and one month in the Hundred alongside.

Brook impressed when he stood in during England’s ODIs against Australia last September, and has other leadership experience with England Under-19s, Yorkshire and Northern Superchargers. At 26, with no secondary skill to worry about in the field and a clear injury history, it could be the making of him. If anyone can cope with the workload, it is Brook.

But that does not make this a straightforward decision. Brook averages nearly 60 in Test cricket but is yet to nail international white-ball cricket: he has now been to four ICC events and his only match-winning innings was against Namibia. He has struggled badly in 2025, with 188 runs in 11 hits – and a solitary half-century – since England arrived in India at the start of the year.

Most pertinently, Brook’s schedule is already packed. England’s next ODI series starts four days after the IPL final; if Brook’s Delhi Capitals are knocked out early, he will likely play a Test against Zimbabwe instead. McCullum insists England’s fixture list is easing, but they still have 11 Tests, 27 bilateral white-ball fixtures and a T20 World Cup scheduled in the next 12 months.

The risks, therefore, are two-fold and interdependent. One, Brook will not have the same single-minded focus on white-ball cricket as Morgan and Buttler, risking selling England’s ODI and T20I teams short. And two, there is a risk that Brook will spread himself so thin that his batting will suffer, to the detriment of England’s Test team.

Those drawbacks were already evident last summer, when Brook was the only player to play throughout the T20 World Cup and England’s home Tests and finished that stretch looking uncertain of his batting tempo after some mixed messaging. It culminated in his leanest full Test series yet, averaging 30 against Sri Lanka as their seamers bored him out at The Oval.

McCullum’s appointment as all-format coach relied on the premise that England’s fixture list is becoming more manageable after years of post-Covid backlog, and should ensure consistent messaging for multi-format players. But surely England will not want Brook to be playing T20Is in New Zealand in the build-up to next winter’s Ashes: if he is appointed, he will need a deputy.

McCullum ruled nothing out on Saturday. He was open-minded about the prospect of appointing separate 50-over and T20 captains – “If it’s the same person, then great; if it’s two different people, then great too” – or even bringing in a leader from outside the current squad. “I’ll get home in the next couple of days and start having conversations with Rob Key and the guys at the ECB about who is the right person for us to put in that position.”

Perhaps the most important conversation of all will be with Brook. England play more Tests than anyone else, and almost as much limited-overs cricket: in the last decade, at least one format has had to give way for each of their permanent captains. Before McCullum and Key decide Brook is their man, they must ensure he feels ready to take on an unprecedented challenge.

Key will know that he has taken his eye off the ball with England’s white-ball teams. He backed Buttler to continue after last year’s T20 World Cup and extended McCullum’s contract to cover all formats; now, Buttler has resigned and McCullum has overseen 10 losses in 11 games. If another appointment goes wrong, it will be Key himself whose position is under the most scrutiny.

He must also bear much of the responsibility for the predictable make-up of England’s bowling attack, which conceded 57.46 runs per wicket and went for 6.8 runs per over at the Champions Trophy. Supported by McCullum, Key has pushed for a stock of all-format, high-pace seam bowlers, but the move backfired: Brydon Carse and Mark Wood both leaked runs, then went down injured.

Both McCullum and Key were riding high two years ago, fuelled by England’s daring cricket and strong results in the early days of the Bazball era and – in Key’s case – their 2022 T20 World Cup triumph. But their stock has fallen since, with significant responsibility for England’s mid-table finish in the World Test Championship table and their disastrous white-ball run this year.

It leaves the pair facing a huge judgement call when it comes to Brook. As England’s best young batter, Brook’s output will go a long way towards dictating their success in both next winter’s Ashes and the T20 World Cup that follows straight after – two series on which Key and McCullum know that their legacies depend.