Rashid, 38, has been England's first-choice white-ball spinner for over a decade, and has played 159 of their 186 men's ODIs since June 2015. He has occasionally missed games due to injury - including September's ODIs against Sri Lanka - but has only rarely been left out when picked in a squad, most recently against Australia immediately after the 2022 T20 World Cup.
England confirmed that Rashid was fit and available for Thursday's match in Rawalpindi, but they opted to pick Liam Dawson, the left-arm spinner, ahead of him for tactical reasons. Rashid was expensive in England's defeat to Pakistan on Tuesday, taking 1 for 55 from eight overs, but it was still unusual to see him in a high-vis vest in the dugout.
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Brook, who made a brilliant double-hundred yet ended up on the losing side thanks to Sri Lanka's fearless run chase, said that Dawson's performance - he took 1 for 71 from 10 overs, the tightest return of any England bowler - vindicated the decision to leave Rashid out, despite the result.
"[Even] in hindsight, I don't know," Brook said. "We brought Daws in for Rash, and I think Daws was probably the pick of the bowlers. He kept the stumps in play for the majority of the game and I thought Daws was probably our best bowler today."
Rashid has had a relatively quiet year in ODIs, taking 12 wickets in his past 10 games at an average of 39.58 and an economy rate of 6.01. Speaking on Sky Sports after Thursday's game, former England captain Nasser Hussain suggested that he had "just dropped off" and that Brook's team need Rashid back to his best ahead of next year's World Cup.
"In the last two or three years, if you look at all the tables and all the stats, England have struggled to take wickets in the middle overs," Hussain said. "That, for me, is a box that England need to tick. Obviously you'll get [Jofra] Archer back and that will help [provide] a little bit of firepower. Maybe Adil's just come off his best as well, a little bit. He was left out today.
"I think today showed the need for Archer, a bit of firepower and the need for Adil Rashid to get back to his best because we all big up Adil, but he's just dropped off - maybe because of injuries, but we need Adil Rashid to get back to his best."
Rashid may well return to the England side against Pakistan on Sunday as the tri-series moves to Lahore. They have also brought Mason Crane on tour with them, who was preferred to Rehan Ahmed as the back-up legspinner but is yet to feature.
Either way, England will need to bowl significantly better than they did on Thursday, as Sri Lanka pulled off the second-highest run chase in ODI history with a ball to spare.
"We can take a lot of learnings from that game," Brook said. "The bowling, we probably didn't execute - well, we definitely didn't execute what we were trying to do for long enough. We wanted to keep the stumps in play as much as we possibly can. We were probably erring on the shorter side, which played into their hands."
Brook also insisted that his own performance - he became England's first double centurion in ODIs, and broke Shubman Gill's record for the format's quickest-ever double - was irrelevant given the result.
"I always say I want to make match-winning contributions and have a match-winning impact," he said. "Unfortunately, I've got 200 and it's not been match-winning - so in my eyes, it doesn't matter."