Key made clear when he was appointed England's managing director two years ago that his decision to split the coaching roles owed to pragmatism, rather than principle. "You get a better list of candidates nearly all of them are like, 'I wouldn't be going for this if it was one [role],'" Key said. "You want the best people, and you build the structure around them."

In effect, that is exactly what Key has tried to do with this gambit. There has been a complete transformation in England's Test cricket under McCullum: he has overseen 19 wins, eight defeats and a single draw, and has given them a clear identity that they were sorely lacking. Key's belief is that he can have the same effect on a stagnant white-ball set-up.

It is a call which highlights the importance of perception. England have played some brilliant Test cricket under McCullum but are yet to win a series against the world's top two: they drew with Australia last summer, and were heavily beaten in India earlier this year. Still, the sense remains of a team on the up - not least after five clinical wins this summer.

The pitfalls of McCullum's new role are hidden in plain sight. Only India play more men's international cricket than England, and England play more Tests than anyone. Their ludicrous fixture list is encapsulated by the 24-hour turnaround between the scheduled fifth-day finish of the upcoming third Test against Sri Lanka and the start of a T20I series against Australia.

Key insisted that the "constant clashes" between formats "are easing, starting from January" but the volume of cricket scheduled is still substantial. Expect McCullum to have few qualms about handing the reins to one of his assistants for several bilateral series over the next three years - as Rahul Dravid did throughout his tenure with India.

The trouble lies further ahead, with a dilemma that is familiar to England's captains and coaches across the past two decades: how can you plan for both an Ashes series, and a World Cup straight after it? It is akin to a tennis player winning Wimbledon and jumping on a plane to Flushing Meadows, or an athlete being asked to compete in a World Championships immediately after an Olympic Games.

In both the 2025-26 winter and the 2027 summer, McCullum will need his players to perform at their best throughout a four-month period - a scenario in which England have always struggled. Nasser Hussain (2003), Michael Vaughan (2007) and Andrew Strauss (2011) all led depleted squads to 50-over World Cups which were staged immediately after Ashes tours, with predictable results; Buttler faced a similar situation last year, with England's multi-format players underperforming in India.

Buttler was among the players who struggled five years ago when the turnaround was flipped. England's 2019 World Cup triumph, the culmination of their white-ball revolution under Eoin Morgan's captaincy, left players physically and emotionally drained yet was swiftly followed by a home Ashes series: England snuck a 2-2 series draw, but were outplayed.

As Australia's fine record at World Cups can attest, scaling twin peaks in quick succession is by no means impossible: only three members of their side that beat India in Ahmedabad last year did not feature on the Ashes tour which preceded the World Cup. Andrew McDonald oversaw success on two fronts, something which has eluded England's coaches.

More immediately, McCullum's task will be to inject some energy into a white-ball set-up which felt desperately low on it by the time they were knocked out of June's T20 World Cup. He will watch from afar for their next 14 games, with Marcus Trescothick in interim charge, then will have a quick tour to India to make his mark before February's Champions Trophy.

The biggest unknown for McCullum will be his relationship with Buttler. Buttler idolised McCullum as a player but has never played or worked with him, and his captaincy is under scrutiny after the manner of England's semi-final defeat to India in Guyana. Where Buttler had the balance of power over Mott, McCullum should be strong enough to take the lead.

England believe they have a new core of young players who can become regulars across formats over the next decade: this includes Harry Brook, Jamie Smith and Gus Atkinson, who have all showcased their attacking style in McCullum's Test team. This is simply the latest step in Key's attempts to bridge the gap between the two codes - which had been in danger of becoming a chasm.

It was barely two years ago that McCullum made clear to Key that he had no interest in taking on England's white-ball teams, after Morgan had transformed them into not only regular semi-finalists but trendsetters in the global game. Now that they are lagging behind, he finds himself in charge of another revival.