The defending champions slumped to a third defeat in four games in the group stage of the tournament on Saturday when they suffered a record 229-run defeat to South Africa, with Buttler admitting his regret at opting to field first in Mumbai after winning the toss.

Former England Test skipper Root is fully behind the wicketkeeper's approach to the white-ball captaincy though and put the onus on him and his team-mates to up their performances after being left on the brink of elimination with five games of the group stage still to play.

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"He knows what he's doing and sometimes as captain you need your players to step up and perform," Root told Sky Sports.

"You can make all the right decisions, but if the guys aren't putting in the right performances, then there's only so much you can do as a leader.

"He's led very well over the last little while and since I've been back involved in the white-ball stuff.

"I don't think there's any worry there we're not getting the most out of Jos Buttler as a captain, as players we've got to stand up and do our bit in putting what we're required to do as individuals on the field and bringing that together as a collective to win games of cricket."

Root was England's leading run-scorer as they triumphed in the 2019 World Cup but was also part of the squad which suffered a humbling group-stage exit four years earlier and has seen plenty of ups and downs for the national team across all formats since making his Test debut in 2012.

He is in no doubt the current squad is among the best he has been a part of, despite their difficult start to the defence of their title in India, and feels the fact they are in a win-or-go-home situation brings a level of clarity which suits them.

"I've played in a number of different England teams - good ones, bad ones - and this is one of the best ones," Root said.

"This is a very together team and we know what we need to do. If anything, this white-ball team over an eight-year period likes very simple messaging and has responded well to it.

"We have some very simple messaging right in front of us now: We have to go out and we have to win.

"In some ways it unshackles us, it frees us up to what we have to do, so now we have to go and do it."

Root does not see England's struggles at this World Cup as a lack of ability in the squad either, although he admits he is at a loss to explain exactly why they have not performed to the levels they have previously showcased.

Instead, the right-hander, who is averaging 43 with the bat so far in this tournament with 177 runs and two half-centuries to his name, is urging England to keep doing the hard work in practice and have faith they will come good for the showdown with Sri Lanka in Bangalore.

"Previous campaigns where we haven't fared well, it's been because we're not a strong enough team and we haven't been able to compete on class, but I don't think that's the case with this group," Root said.

"Sometimes there's not just a magic pill you can take overnight and it's perfect, you've just got keep putting in the hard work, keep readying the best you can, and understand there's so much talent in this group and so much ability, we will be able to do that.

"All we can focus on is putting that performance in the next game."

Watch England's must-win game against Sri Lanka at the Cricket World Cup live on Sky Sports Cricket and Sky Sports Main Event on Thursday, October 26 from 9am (first ball 9.30am). Also stream the entire tournament and more with NOW.