Root rates Roy one of at least six current England batsmen – he includes himself, with some reservations – who have the ability to become the first from this country to hit a one-day international double-century.
He saw enough too, in his century stand with the opener at The Oval on Wednesday, to know the privileged position as non-striker when Roy is in full flow does not come without its dangers.
Roy’s 162 was six short of a new national record individual ODI score, as England sealed the Royal London Series against Sri Lanka with a match to spare.
The prospect of rejoining Roy in the middle today in Cardiff, where England will attempt to finish 3-0 winners, is one that fills Root with excitement – and a little trepidation.
He can fully understand too why umpire Bruce Oxenford, standing again this weekend, has taken the precaution of carrying a ‘Captain America’-style shield with him in this series.
Roy’s prolific, and brutal, form has brought him 279 runs and just one dismissal in his last three innings.
At nets in Cardiff, it took him 12 deliveries to break a new bat – playing a forward-defensive.
“It’s amazing to watch, from 22 yards away,” said Root. “It’s a strange, on edge but relaxed feeling, when you see the ball flying to all parts.
“There were a couple the other day he hit back down the ground that I thought – if they were a couple of yards higher – I could be in a spot of bother.”
Root puts Roy’s opening partner Alex Hales and a middle order of Eoin Morgan, Yorkshire’s Jonny Bairstow and Jos Buttler in the same bracket.
Roy and Hales’s unbroken stand of 256 at Edgbaston last week is a case in point.
“It’s what you want to see, guys getting in and breaking records,” said Root. “With what we had to come in afterwards as well, it is quite frightening what we could be capable of.”
Roy was at it again five days later, further evidence for Root that an Englishman may soon add to the list of just five batsmen to have hit an ODI double-hundred.
“Definitely, there are a number of players in our team that would be capable of getting scores over that,” said Root.
“I think anyone in the top six, really. I’m not sure I’d be able to score that quickly – but you never know.”
He has no such qualms about Roy’s capabilities.
“(Jason) was out in the 38th over ... with 160 under his belt, not really looking like he was trying to hit every ball for four and six and still scoring as quickly as he was,” said Root.
“That’s quite scary to see, isn’t it?
“It’s a great ‘scary’ as well.
“When you’re sat at the other end, there is no pressure on you.
“You know the scoreboard is always going to be turning over, and the opposition is always under pressure.”
England’s white-ball cricket has been a revelation since June last year, but even so, Root wants to see more consistency.
“We all know it’s not always going to be that easy, and it’s great to see his work ethic as well ... with the rest of the guys too,” he said.
“It’s infectious, and you all get that bug.
“(But), it’s about being consistent and doing it more and more ... I suppose that’s our challenge.”
Only then, he warns, will England be delivering properly.
“We’ve got all this excitement and flair, ability and potential ... but it’s about winning big trophies, big series ... if we’re going to be serious about contending for Champions Trophies and World Cups,” said Root.
“We’ve got to win series like this, and the ones away from home as well ... not using that old excuse, we’re playing in foreign conditions, we’ve got to make sure we’re matching teams.
“That maturity has come a long way in the last couple of years, and it’s going to have to keep improving if we want to be the best in the world.”
Root’s 65 at The Oval was only his second international 50 of a patchy summer so far, by his high standards.
But he returns this weekend to the scene of last summer’s first Ashes Test win in which he hit a century, and took the catch which ended the match.
“It’s always nice ... to relive what was a really strong start to that series,” he said.
“It would be a fitting way to finish this series if we could do something similar (today).”