A commanding eight-wicket win against dangerous opponents was important in firming up England’s semi-final prospects, though seeing two of their best batsmen limp from the field was a bitter pill to swallow.
Roy hurt his left hamstring sprinting at cover and Morgan headed straight for the pavilion after a back spasm, with neither man taking their usual places in the batting order.
England will be desperate for positive news from the treatment room, but the pair were not missed on the day, a modest chase of 213 proving a walk in the park for Yorkshire’s Root, who opened for the first time in his international one-day career and made 100 not out in 94 deliveries.
Root had already played an unlikely role with the ball, taking two wickets with his occasional off-spin, and the role reversals continued with a Chris Woakes cameo at No 3.
Having never batted higher than seven in his previous 91 appearances, the all-rounder was handed an emergency elevation and chipped in with an assured 40.
The game was brilliantly set up by the bowlers, Barbados-born seamer Jofra Archer and Mark Wood delivering with pace and purpose as they shared combined figures of 6-48.
Archer’s first appearance against the team he had represented at Under-19 level had been much debated, but he took the moment in his stride as the West Indies slid from 144-3 to 212 all out.
All eyes were on the 24-year-old after England opted to bowl first, but first blood went to Woakes, scattering Evin Lewis’s stumps with a toe-crushing yorker in the third over.
Woakes thought he had a second – the prize scalp of Chris Gayle – but was left aghast as a tough chance popped out of Wood’s hands at third man.
Giving Gayle a second chance can be among the costliest mistakes in one-day cricket, and a smooth six down the ground suggested the price tag might be a hefty one. Y
et the burly Jamaican never quite made England pay, making 36 before pulling Liam Plunkett to deep midwicket.
Wood made his own amends, ending a tortured knock by Shai Hope, before Nicholas Pooran (63) and Shimron Hetmyer (39) showed some backbone. Their 89-run stand frustrated England rather than hurt them, but it was as good as it got for their rivals as the innings fell off a cliff inside 15 overs.
In the absence of Moeen Ali, overlooked for the second game in a row, it was Root’s seldom-seen off-breaks that kick-started a collapse of seven wickets for 68.
Having last taken an ODI wicket 18 months previously he took a pair of gentle caught-and-bowled chances, Hetmyer and Jason Holder succumbing.
The door was open and Buttler, deputising for the absent Morgan, called on Archer and Wood to kick it down.
An edge from Pooran and a plumb lbw put Archer on a hat-trick, but he had to wait before making Carlos Brathwaite his third victim of the day.
Wood, who passed a morning fitness test on his left ankle, was equally impressive, hustling Andre Russell with a short ball and cleaning up Shannon Gabriel.
Root replaced Roy at the head of the innings and joined fellow Yorkshireman Bairstow in ramming home the advantage.
They took 62 off the first 10 overs, Bairstow taking care of business as usual and Root piercing the infield with a selection of sweetly hit drives.
The West Indies’ attack was dangerous on paper, but a crushing disappointment in the middle, lacking the intensity of their English counterparts and throwing in regular boundary balls.
Even their minor triumphs came at a price, Russell thudding Bairstow in the helmet, but buckling his own knee in the process.
The first breakthrough came with 95 on the board, Bairstow uppercutting Gabriel to third man, leading to the surprise emergence of Woakes at No 3.
Any doubts about the wisdom of the promotion were swiftly dispelled as he got off the mark with a punch for four then pulling Gabriel to the ropes.
A 104-run stand made the result safe, Woakes falling just before the line, but Root safely reached his 16th one-day ton and third on the World Cup stage.
With three wins from four outings, England nudged up to second in the table ahead of inviting games against Afghanistan and Sri Lanka.