The fourth round tie was over as a contest from the moment United took the lead after just eight minutes with the first of two goals by Ryan Giggs, who is not exactly known for his scoring exploits.
Wednesday's home Premiership match against Blackburn Rovers is far more important to Hammers than the result of this game but, for all United's brilliance, the manner and margin of the defeat will only add to Glenn Roeder's concerns.
It was far too easy for the Reds who, after Giggs' first half double, found the net four more times in the second half through Ruud Van Nistelrooy (twice), Phil Neville and Ole Gunnar Solskjaer.
Ten goals conceded in four days, at Charlton and now Old Trafford, has inflicted further psychological damage on an already fragile defence that continues to leak bad goals at an alarming rate.
A two-goal first half deficit flattered the visitors, who had been under the cosh for sustained periods as United stroked the ball around their muddy Old Trafford pitch at will. Roeder dropped Christian Dailly to the bench following the midweek debacle at Charlton but it doesn't matter who lines up in the back four, the same collective and individual defensive failings continue to haunt the Hammers.
No-one's confidence would have been dented more than that of Gary Breen. He was the culprit for the first goal, getting the wrong side of Van Nistelrooy who eased him off the ball and pulled a by-line cross back for Scholes. Although his weak shot was blocked on the line by Ian Pearce (returning to central defence at the expense of Dailly), the ball rebounded to Giggs who, standing all alone just eight yards out, had the time and space to pick his spot.
It was all too easy for the Welsh winger again on the half-hour, when he again took up a central position. While Breen's eyes were fixed on Juan Sebastien Veron, Giggs stood still to receive the Argentine's pass from the left and had enough space to clip a shot that flew in via Breen's left shoulder.
At times it was like a practice match for the rampant Reds, who were given too much time in possession, while their movement off the ball was too much for Hammers to handle. On the rare occasions that West Ham did win the ball, they invariably wasted it with a sloppy pass here and a long, hopeful punt there.
Jermain Defoe was asked to go it alone up front (Les Ferdinand wasn't signed in time to play in the fourth round, while Paolo Di Canio and Fredi Kanoute still hadn't recovered from injuries) but the little striker, who has an impressive scoring record against United, received poor service.
It took Hammers 35 minutes to muster a shot at goal - a Lee Bowyer strike that Fabien Barthez gathered comfortably. Edouard Cisse should have done better on a rare counter-attack but, once again, his pass didn't find Defoe, who had taken up a good position on the edge of the United box.
Strangely, West Ham had three good chances to score in the last five minutes of the half as United relaxed their concentration. Joe Cole, playing in a free role just behind Defoe and working relentlessly, got to the by-line to set up Bowyer for a rasping shot that Barthez parried.
Cole then tested the French keeper with a low drive before Sinclair should have connected with a good Bowyer cross from the right that dropped on the edge of Barthez's six-yard box.
But any hope that West Ham would build on their late first half flurry and turn it around after the break were quickly shattered as United scored twice in a minute at the start. Again, Van Nislelrooy got the better of Breen too easily on the edge of the box, sent both Steve Lomas and Pearce the wrong way and made it three with the outside of his right foot from 12 yards.
There was only weak resistance by the Hammers defence again when Phil Neville - yes, Phil Neville! - was allowed to charge from left-back, past Cisse, collect Diego Forlan's return pass, brush past Lomas and leave Breen on his backside before chipping the ball over James from six yards. All too easy.
It got worse, though, for Breen and Hammers before the hour mark. Once again, Van Nistelrooy got the better of the Republic of Ireland international, after Beckham had nutmegged Pearce. James rushed out to challenge but the ball broke kindly for the Dutchman, who rolled the fifth into the empty net.
It was Scott Minto's turn next to be skinned by Van Nistelrooy, turning in from the right to play in the unmarked sub, Ole Gunnar Solskjaer, who placed his shot under the despairing dive of James to complete the rout after 68 minutes.
Breen's ordeal ended mercifully 10 minutes from time, when he - along with Sinclair and Cisse - were replaced by Dailly and young duo Glen Johnson and Richard Garcia.
Within a minute, Cole almost carved out a goal with a piece of individual brilliance on the edge of the Reds' box. West Ham's best player on a miserable day wriggled through the centre of the United defence and sidefooted a left-foot shot that Barthez parried to his right. Bowyer got in front of Gary Neville and was clearly fouled from behind by the United skipper, but referee Steve Bennett added further insult to injury by failing to award a blatant penalty.
A goal at that late stage would obviously have made no difference to the result but it would at least have given something for the 9,000 visiting fans to cheer. They stuck by the team throughout and are entitled to expect a big improvement against Rovers at Upton Park on Wednesday.
Minute-by-minute:
2 - A sniff of an early chance for the Hammers, as Cole dispossesses Gary Neville on the left and cuts into the box before squaring the ball to Defoe, who in turn sets up Sinclair, whose shot is blocked.
6 - Gary Neville's deep cross is kept in by Giggs, whose low driven cross is well intercepted by a Hammers defender and cleared to safety.
7 - GOAL. Just the start Hammers didn't want, and the goal is once again down to dreadful individual defending. A simple ball down the channel appears harmless, but Breen gets the wrong side of Van Nistelrooy, who out-muscles him and lays the ball back for Scholes. His shot is cleared off the line by Pearce, but Giggs is on hand to collect the loose ball and fire into the net. Manchester United 1 West Ham United 0.
10 - Veron booked for foul on Cole.
13 - Veron's cross from the left is met by Beckham, who rises above Minto at the far post but directs his header wide of the target.
29 - GOAL. United's second arrives after Hammers have ironically enjoyed their best spell of possession so far. Veron gains possession on the left and slides in a pass to the unmarked Giggs, whose first time shot from the edge of the area takes a deflection off Breen and loops up and over James into the net. Manchester United 2 West Ham United 0.
33 - James prevents an immediate third, as he superbly tips Beckham's 30-yard thunderbolt over the bar.
35 - United are threatening to put the result beyond doubt before half-time, and go close again as a flowing move involving almost every member of their team ends with Scholes hitting the post from 12 yards out.
37 - A rare moment of attack for the Hammers, as Bowyer fires in a speculative shot that is easily gathered by Barthez.
43 - Hammers end the half with their best spell of the match, as Defoe releases Cole, whose low cross is almost diverted into his own net by O'Shea. The ball then comes out to Bowyer,whose fierce effort is blocked by Barthez.
44 - Cole goes close this time, firing in a low, skidding shot from the edge of the penalty area that Barthez keeps out with his legs. From the rebound, Bowyer's cross is just inches from the head of the diving Sinclair in the six-yard box.
Added time: 1 minute
Half-time: Manchester United 2 West Ham United 0.
Second Half:
46 - Substitution for Manchester United, Forlan replaces Scholes.
49 - GOAL. The hosts add a crucial third goal with their first attack of the second half, as Van Nistelrooy's looping effort from 10 yards out sails into the far corner. Manchester United 3 West Ham United 0.
50 - GOAL. The afternoon is getting rapidly worse, as Phil Neville plays a one-two with Forlan and skips into the box before clipping the ball over James for a rare goal. Manchester United 4 West Ham United 0.
51 - Substitution for Manchester United, Butt replaces Veron.
52 - Another rare attack for Hammers, as Bowyer finds some space on the left, but curls his shot high and wide.
58 - GOAL. It's getting worse and worse for Hammers, as James' weak clearance is collected by Beckham, who measures a pass through to Van Nistelrooy - running clear with his marker Breen 10 yards away. The Dutchman gets a lucky bounce as he rounds James and slots the ball home. Manchester United 5 West Ham United 0.
63 - Substitute for Manchester United, Solskjaer replaces Beckham.
69 - GOAL. A predictable sixth, from a predictable source, as Solskjaer races on to Van Nistelrooy's pass and fires the ball low past James with his first real touch of the game. Manchester United 6 West Ham United 0.
71 - Defoe booked for foul on O'Shea.
80 - Triple substitution for West Ham, Garcia replaces Cisse, Dailly replaces Breen and Johnson replaces Sinclair.
81 - Hammers see the chance of a consolation goal go begging as Cole, one of the only players to show the required level of commitment and desire for West Ham, shows excellent determination to burst through three challenges into the penalty area but sees his point blank effort blocked by Barthez. Bowyer appears to be fouled as he collects the loose ball, but referee Bennett waves play on.
84 - Minto booked for foul on Solskjaer.
Added time: 1 minute.
Full time: Manchester United 6 West Ham United 0.