PHIL Jones and Wayne Rooney headers either side of the break set up Pepe Mel's first Hawthorns defeat.
In a tight first-half, centre-half Jones broke the deadlock by nodding home Robin van Persie’s 34th-minute free-kick.
Albion rallied and had their moments but the game was effectively over in the 65th minute. While skipper Chris Brunt was off the field receiving treatment, United took full advantage, with Rooney heading home Rafael’s centre.
The champions’ third and final goal – courtesy of sub Danny Welbeck seven minutes from time – put an unfair reflection on the scoreline.
Seventeenth entertained seventh in the day’s early kick-off.
Mel made four changes from the draw against Fulham a fortnight ago, with fit-again Claudio Yacob, Steven Reid, Zoltan Gera and Victor Anichebe coming in for benched trio Saido Berahino, James Morrison and Thievy and the injured Billy Jones.
Before the action got underway, a minute’s applause was impeccably observed by both sets of supporters in memory of the club’s England international right-back Stan Rickaby, who passed away recently aged 89.
Albion started brightly and their non-stop pressing of United’s backline gave the visitors a few nervy during the opening minutes.
It took a timely challenge from Liam Ridgewell inside the box in the 11th minute to prevent Marouane Fellaini testing former United keeper Ben Foster from Adnan Januzaj’s cross – before the Baggies were back on the attack.
Anichebe collected a loose ball just inside the Albion half before powering past Chris Smalling into the visitors’ box. It took a combination of Smalling and central defensive partner Jones to finally wrestle the ball from the former Everton striker in the nick of time.
United threatened again from Rooney’s 16th-minute corner and the towering Fellaini’s header struck Brunt before being hacked off the line by Youssouf Mulumbu.
Although chances were few and far between, it was turning into an intriguing encounter and Rafael had to produce a vital block inside his own box to prevent Brunt’s shot from troubling David de Gea, after neat build-up play by Mulumbu.
Then, on 28, de Gea had to make a smart save at his near post to keep out Morgan Amalfitano’s flick-on from Brunt’s deflected cross.
But Foster had to produce a truly world class save less than a minute later to keep the Baggies level.
The unmarked Rafael looked certain to score as he leapt to meet Januzaj’s sublime cross at the back post, only to see his header somehow diverted onto the bar by the Albion No.1.
But Foster could do nothing about United’s opener on 34. After Rafael won a free-kick on the corner of the Albion box, van Persie’s perfectly-flighted delivery was glanced home by the unmarked Jones from ten yards.
Albion regrouped and went close themselves from a set piece on 38. Brunt’s corner was nodded forward by Jonas Olsson and Yacob out-jumped Rooney but saw his header loop onto the roof of the net.
The hosts suffered an injury blow on 40 when Yacob was unable to continue after a late challenge from Fellaini.
The Argentine’s replacement, Morrison, almost made a telling contribution with his first touch. The Scotland international fed Anichebe who held off Jones before firing inches wide of the far post.
Moments later, Foster had a lucky escape when, with van Persie bearing down on him, he failed to control a high diagonal pass with his left foot just outside his box and the ball struck his trailing arm. United appealed for a free-kick but although the frame of Foster’s body was outside the penalty area, his arm looked level with the edge of the box.
After the interval, both sides recorded shots from distance during the early exchanges, but Januzaj and Morrison were well off target.
Gera produced a moment of magic on 53 when he turned neatly on the edge of the box and exchanged a smart one-two with Anichebe before shooting straight at de Gea from 15 yards.
The home hordes were baying – albeit in vain – for a red card on 55 when van Persie, who had only been booked five minutes earlier, lunged in on Reid. TV replays suggested referee Jon Moss had been lenient.
But The Hawthorns was now rocking and only Juan Mata’s last-ditch clearance, which prevented Brunt tapping home Anichebe’s centre, denied Mel’s men a leveller.
The action swung to the other end and Ridgewell had to produce an equally important block to divert Fellaini’s shot away from goal from van Persie’s cross.
Mel made his second change on the hour-mark, replacing the injured Reid with Stephane Sessegnon, who was making his comeback from a two-month groin injury lay-off.
There were more worrying injury signs for Albion when Brunt required lengthy treatment after falling awkwardly on the edge of this own box.
While the skipper was off the pitch, United made their extra man advantage count with deadly effect. Rafael crossed from the right and Rooney ghosted in at the far post to head home.
Brunt was unable to continue and was replaced by Berahino.
Albion did not throw in the towel and de Gea showed terrific reflexes to push away Gera’s acute drive at his near post after another smart exchange with Anichebe.
Moments later, Anichebe spun his marker 18 yards out before firing wildly wide.
Albion were piling on the pressure and Rafael risked life and limb in getting in the way of Berahino’s 15-yard drive.
But any lingering hopes of a late fightback were finally extinguished on 83 when sub Welbeck surged onto Rooney’s through-ball before hooking the ball past Foster.
ALBION (4-2-3-1): Foster; Reid (Sessegnon 60), McAuley, Olsson, Ridgewell; Yacob (Morrison 40), Mulumbu; Amalfitano, Gera, Brunt © (Berahino 65); Anichebe. Subs not used: Myhill (gk), Thievy, Vydra, Dawson.
UNITED (4-2-3-1): De Gea; Rafael (Vidic 87), Jones, Smalling, Evra ©; Carrick, Fellaini; Mata, Rooney, Januzaj (Kagawa 76); van Persie (Welbeck 63). Subs not used: Lindegaard (gk), Giggs, Young, Fletcher.
GOALS: UNITED – Jones (34), Rooney (65), Welbeck (83).
BOOKINGS: ALBION – Anichebe (unsporting behaviour 90). UNITED – Januzaj (deliberate handball 13), van Persie (foul 50).
REFEREE: J Moss (West Yorkshire).
ALBION STARMAN: Victor Anichebe.