Andrew Miller is UK editor of ESPNcricinfo. @miller_cricket
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Roy, 33, was an integral member of the England squad that won the World Cup in 2019, and had been named in England's provisional squad for the defence of their title, ahead of their four-match series against New Zealand this month.
However, he was unable to feature in any of the four games after suffering a series of back spasms. In his absence, Roy's rival for the opener's role, Dawid Malan, made an unequivocal case for a starting role in the tournament, finishing as Player of the Series with 277 runs at 92.33 in three games, including a matchwinning century at Lord's.
Consequently, it will be Malan and Jonny Bairstow who will open the batting for England when their tournament gets underway against New Zealand in Ahmedabad on October 5, with Brook - the notable omission from their original 15-man squad - preferred to Roy as the squad's reserve batter.
Brook himself did not make a convincing case for his inclusion in the course of New Zealand series, managing a total of 37 runs from 68 balls all told, including two stints as an opener following Roy's original back spasm. He has also featured in just six ODIs in his career to date, having debuted in the format in South Africa in January.
However, Brook's outstanding displays across formats this past year - most notably in Test cricket, where he has made 1181 runs at 62.15 and a strike-rate of 91, but also with a sensational 41-ball century for Northern Superchargers in the Hundred - have convinced the selectors that he is an asset they cannot do without.
"We have selected a squad we are confident can go to India and win the World Cup," Luke Wright, England's national selector, said. "We are blessed with an incredibly strong group of white-ball players which was underlined by the performances in the series win against a very good New Zealand team.
"The strength of the group has meant that we have had to make some tough decisions on world-class players with Jason Roy missing out and Harry Brook coming into the squad."
In the wake of the Lord's match on Friday, Jos Buttler had hinted that Roy could be added to the ODI squad to face Ireland next week, in a bid to get some game-time ahead of the tournament, adding that loyalty to proven performers was a central tenet of England's white-ball philosophy.
"If you ask people to play in an aggressive way, then leave them out as soon as it goes wrong, it sends the wrong message," Buttler said. "You can't just pick a World Cup squad from these four games."
However, Roy's returns had been on the wane for some time, even before his back problems forced the selectors' hands. After making his ODI debut in 2015, soon after England's debacle in that year's World Cup, he averaged 42.79 with a strike rate of 107.40 in the four years up to and including their thrilling victory in the 2019 final at Lord's.
However, despite being England's most regular ODI selection in the four years since, Roy's average in that time had dipped to 31.78 with a strike-rate of 98.99. He displayed his tenacity earlier this year in making two hundreds in contrasting conditions in Bloemfontein and Mirpur, but by then he had already been axed from England's T20I plans, and missed their triumphant campaign in the T20 World Cup in Australia last November.
The decision to omit Roy almost certainly brings an end to his international career. He had already indicated that the World Cup would be his international swansong, and having negotiated an early release from his ECB central contract this year, in order to play for Los Angeles Knight Riders in the inaugural season of Major League Cricket in the USA, a stint on the T20 franchise circuit would appear to be his next career move.
Either way, his omission is clearly not one that England will have taken lightly. In facing the first ball in each of his 116 ODIs, he was charged with setting his team's aggressive tempo from the outset of every innings, and did so in groundbreaking fashion, including with a highest score of 180 from 151 balls at Melbourne in 2018 that was, until Ben Stokes' innings at The Oval last week, England's record individual score in ODI cricket.
He will also remain synonymous with England's 2019 World Cup victory, in particular for his blazing knock of 85 from 65 balls in the semi-final against Australia at Edgbaston, and for sending in the shy from deep midwicket that secured England's ultimate victory in the tied Super Over at Lord's against New Zealand.