Erik ten Hag insisted there would be no hangover following Manchester United's Carabao Cup final triumph.
And although they left it late, until the 90th minute in fact, United ultimately proved their manager right, to advance to the quarter-finals and keep their Quadruple dream alive. United's match-winner was 18-year-old sensation Alejandro Garancho, whose curling effort just as added and extra-time beckoned, sealed the win for Ten Hag's side.
The Red Devils looked to be heading for defeat, after Said Benrahma gave West Ham the lead after the break, but an own goal, Garnacho's late strike and another from Fred in added time sealed the comeback win.
Ten Hag can do no wrong right now. After ending United's six-year wait for a trophy with Sunday's Carabao Cup final triumph, the 53-year-old Dutchman kept them on course for another piece of silverware in his debut season in charge.
The Hammers had to wait until the 23rd minute for their first opportunity, Said Benrahma putting Michail Antonio throgh on goal, the forward having peeled away from Harry Maguire and Victor Lindelof.
United's second-choice central defensive pairing were left trailing Antonio and were indebted to David De Gea – fresh from a club record 181st clean sheet on Sunday - who blocked the West Ham forward's angled effort.
David Moyes' side continued to threaten, primarily on the counter-attack, with Benrahma in particular tormenting them, McTominay earning a booking just after the half-hour for chopping him down, joining skipper Harry Maguire, booked minutes earlier for scything down Antonio.
West Ham ended the first-half on top, Maguire's booking leaving him reluctant to challenge Antonio, who had greater freedom to drop deep, run in behind and trouble United's defence, with Benrahma allowed to run amok in the first 45 minutes.
McTominay was replaced at the break by Casemiro, the experienced Brazil star immediately giving United more control and authority in midfield, but it was West Ham who produced the first chance of the second-half in the 50th minute, De Gea diving low to his right to palm away a Soucek header.
West Ham took the lead four minutes later, Benrahma scoring with a superb strike into the top right corner. United claimed the ball had gone out of play with Soucek, who fed it to Emeson Palmieri to set up the strike, but the goal stood after a check.
The lead could have been doubled a few minutes later, when Pablo Fornals steered the ball across the face of the United goal, but Soucek and Declan Rice, both lurking at the far post, were unable to turn it into the net.
With United pushing for an equaliser, they inevitably left themselves open at the back, De Gea coming to their rescue in the 62nd minute, when Antonio tried his luck with an angled effort which the Spaniard did well to tip away.
Just as he did at Wembley in the Carabao Cup final win, Casemiro scored with a glancing header from a free-kick, but the 71st minute goal was ruled out for offside after a VAR check, a decision greeted with jubilant cheers from the 6,000 travelling West Ham fans.
But their joy was short-lived when Hammers defender Nafyd Aguerd headed a Bruno Fernandes corner into his own net in the 77th minute, to make it 1-1. Suddenly, it was United in the ascendancy, with Garnacho putting them in front with a majestic curling effort in the final minute. And the scoreline was given an extra gloss when Fred made it three with one of the final kicks of the game.
