Matt Roller is an assistant editor at ESPNcricinfo. He tweets at @mroller98
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While that innings came in defeat, it provided a eureka moment for Rajasthan. Buttler continued in that role for his final five games of the season, hitting 361 runs in five innings at a strike rate of 155.60; over the course of that IPL, only the pinch-hitting Sunil Narine scored faster in the Powerplay.
Buttler has opened in 28 of his 29 T20 innings since his promotion in Delhi, averaging 46.88 in the role while striking at 153.72. He is belligerent against seam (70.36 average, 167.50 SR) and still impressive in his supposedly weaker suit, striking at 132.83 and averaging 32.64 against spin. The premise underlying the move was simple - to ensure the side's best player faced as many balls as possible - but proved to be effective.
There was once a debate to be had about Buttler's best role for England in T20Is, but those numbers offer little room for negotiation. Like most teams, England find themselves with a surfeit of top-order options and fewer options in the 'finisher' role - but they need only ask their opponents this week about the risk of complication.
In AB de Villiers' T20 international career, he averaged 26.12 with a strike rate of 135.16, compared to 39.95 and 151.23 respectively in the IPL. His problem for South Africa was a lack of clarity in his role: as Freddie Wilde and Tim Wigmore point out in Cricket 2.0, de Villiers' longest stretch of consecutive innings batting in the same position was a six-game run at No. 4 back in 2010.
His struggles in T20Is were epitomised by the defeats that effectively knocked them out of consecutive World T20s. Against India in the 2014 semi-final, he arrived at the crease with only 37 balls remaining in the innings, scant time to make a meaningful impact on the game, and South Africa failed to defend their 172. In 2016, after starting the tournament as an opener, he strode in at 20 for 3 after three overs in a must-win Super 10 game against West Indies, with the majority of the side's best players already dismissed.
Similarly, while England might not have a batsman with the same profile of Hardik Pandya or Andre Russell - a fast-starting hitter, best used at the end of an innings - their captain, Eoin Morgan, is in an outrageous run of T20 form, with a death-over strike rate of 225.83 over the last two years. With Moeen Ali best suited to batting in the middle overs, where his mastery against spin comes to the fore, and Ben Stokes back in the side, it seems those three will fill in at Nos. 4-6 by the time this winter's T20 World Cup arrives, with Buttler at the top of the order.
But there is less clarity regarding who should partner him. Having been an all-format regular since - and, in part, due to - his stellar IPL run in 2018, Buttler has only opened the batting six times in T20Is, and has been paired with Jason Roy at the top of the order in each of them. But quietly, Roy's place has become insecure over the past 18 months, in part because his focus has been elsewhere - he has only played 11 T20s since the start of 2019 - but also due to the fact his record in the format pales in comparison to his superlative 50-over one.
Since the start of 2017, Roy averages 24.71 and strikes at 148.25 in all T20 cricket: a decent record, but hardly an overwhelming case for inclusion ahead of some of his competitors. Most alarming, though, is his record against spin, and legspin in particular: in that period, he strikes at 138.50 at averages 20.83 against all slow bowlers; against legspinners, he has faced 99 balls and been dismissed ten times, leaving him with an average of just 11.80. With the further consideration that Buttler's record against pace will likely lead teams to use their spinners in the Powerplay against England, that is a cause for concern.
There are reasons to be wary about those figures: the sample size is relatively small, Roy's recent opportunities in T20 have been somewhat disjointed, and he has a comparatively strong record against spin in 50-over cricket. But given the strength of the chasing pack, he could certainly do with a run of scores in this series.
The alternative candidates are plentiful, and all of them will have opportunities before the World Cup in October thanks to the sheer number of games they will play. Since Buttler's last T20I, in October 2018, England have used Jonny Bairstow, Alex Hales, Ben Duckett, James Vince, Dawid Malan and Tom Banton at the top of the order, while Liam Livingstone, Phil Salt and Joe Denly are further options. After seeing the quality of the queue ahead of him, Joe Root appears to have accepted he is unlikely to make England's squad for the tournament, admitting last week that he is "probably not in the best XI".
Much has been made of Malan's superb T20I record - including by the man himself, who wrote in a recent Sky column he didn't understand "how you can be under pressure" with an average of 57.25 and a strike rate of 156.31 - but the fact he is a slow starter counts against him. Since 2018, he has scored at 87.54 in his first five balls and 104.84 in his first ten, suggesting he might not be the man to maximise the fielding restrictions; and Morgan's thinly-veiled criticism about his failure to run a bye off the last ball in Napier in order to boost his average suggests his character might count against him.
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Bairstow's stunning form in last season's IPL makes him a tempting option to open with Buttler, not least given his strength against spin complements Buttler's relative weakness against it, but he may be used best as a No. 3: he starts his innings quickly, scoring at 123.40 in his first five balls and 142.28 after ten, demonstrating his ability to both make good use of fielding restrictions in the first six overs and accelerate through the usual post-Powerplay lull. There is a temptation to suggest that England should put their 50-over pair back together, but Buttler's record in the first six overs demands otherwise.
Banton's emergence last summer, and his strike rate of 192 against spin in the Powerplay, makes him a clear candidate despite him missing out on this squad, but in truth, that spot is up for grabs with so much time before the tournament itself.
To take Banton as an example, it is possible that he could play more than 40 domestic short-form games before the World Cup, with stints in the Pakistan Super League, the IPL, the Blast and the Hundred lined up, despite the fact England only have another nine T20Is scheduled between the end of this series and the start of the tournament. After that volume of cricket, the picture will become much clearer.