While Roy missed each of the four matches against the Black Caps, fellow opener Malan passed fifty in all three he played, topping out at 127 from 114 balls in the final fixture at Lord's.

The left-hander had fallen four runs short of a fifth ODI century across London at The Kia Oval on Wednesday but completed the milestone two days later, etching his name onto the honours board and, surely, into England's XI for the World Cup opener against New Zealand on October 5 as well.

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Sky Sports Cricket's Nasser Hussain said of Malan: "He is disciplined and orthodox - a machine. He is very consistent, the glue that allows everyone else at the other end to come out and be positive.

"He likes to bat through [the innings] but his strike-rate is up at 95, 96 so he doesn't plod along. He puts anything short and wide away and is in phenomenal nick."

Brook's hopes fade as World Cup picture becomes clearer

Malan had seemed - externally at least - as one of the players at risk if England chose to select Harry Brook in their final World Cup 15 having, to much consternation, left the Yorkshireman out of their provisional squad for the tournament in India.

The picture became even hazier when Brook followed a 41-ball hundred in The Hundred by impressing for England in the T20I series against New Zealand, scoring an unbeaten 43 from 27 balls and then 63 from 36. The clamour for his inclusion had reached fever pitch.

But it has quietened now, with Brook's returns in the ODI series after being called up as batting cover, initially for Jonny Bairstow (shoulder niggle) and Malan (paternity), a meagre 25, two and 10.

The first two of those knocks came in an unfamiliar opening berth, the third from his more customary spot in the middle order. Wherever he batted, he was largely ineffectual.

As disappointing as that is for Brook, his lack of runs, Malan's truckload of them, Roy's back spasms and Liam Livingstone's impact with bat and ball have crystallised matters.

If there was doubt about the make-up of England's top six to face New Zealand at the World Cup, then there probably isn't any more. It would be a surprise if it was not Malan, Bairstow, Joe Root, Ben Stokes, Jos Buttler and Livingstone in that order.

'Roy has to play in Ireland series to make World Cup'

Even if Roy overcomes his back issues and finds form - there is talk that he could be added to the squad for the three ODIs against Ireland to get game time - it will be incredibly hard to overlook a player in Malan who has just reached 1,000 ODI runs in 21 innings and averages 61.52 in the format.

In his last 15 ODIs, he has scored five hundreds in five different countries.

Malan appears to have gone from back-up to first choice, with Roy, through no fault of his own, going the other way - and possibly even worse.

England's 2019 World Cup-winning captain Eoin Morgan says it is imperative the Surrey batter now features against Ireland, with that series starting at Headingley on Wednesday.

"The biggest concern now is Roy's level of fitness," Morgan said of a player who scored 443 runs in eight matches at an average of 63.28 as England won the World Cup four years ago.

"He has to play in the Ireland series as you can't sit in a selection meeting with any amount of confidence and pick him for the World Cup at the moment.

"He has to go through a certain level of game time, recuperation and then possibly another game to try and prove his fitness."

Atherton: Back spasms are difficult to deal with

Morgan's fellow Sky Sports Cricket pundit Michael Atherton added: "I think the [Roy situation] is concerning for a number of reasons, including the lack of 50-over cricket he has had since March, when he did well in Bangladesh.

"He has played a bit of Vitality Blast, a bit of Major League Cricket in America, a bit of Hundred, so this series would have been the opportunity to reacquaint with the rhythms of 50-over cricket which are markedly different to T20, particularly at the top of the order.

"The other concern is the type of injury. I speak from experience when saying back spasms are difficult to deal with because they suddenly come on and when you get them they are in the back of your mind. You don't have full confidence in your body.

"The nature of this tournament in India means you are flying from venue to venue. Every day is a flying day, different hotels, different beds, lots of travel, which is not good for that type of injury.

"England have yet to name their final squad and, who knows, Roy might not make it."

Brook could be the beneficiary if Roy is deemed not fit - but Malan now needs no help to make the final 15 or indeed the XI.

Watch England's three-match ODI series against Ireland live on Sky Sports, starting with Wednesday's opener at Headingley. Coverage begins at 12pm ahead of the first ball at 12.30pm.

Watch the Cricket World Cup live in full on Sky Sports between October 5 and November 19 with England vs New Zealand kicking off the competition in a repeat of the 2019 final.