Wayne Rooney cuts inside to fire the opening goal past Scott Carson
Wayne Rooney sparked Manchester United into life against a limited West Brom as the champions moved ominously up to fourth in the Premier League table.
The visitors battled hard early on while Rooney had an effort disallowed.
But the in-form England international cut inside Ryan Donk to fire a superb opener on 55 minutes before setting up Cristiano Ronaldo for a simple second.
Dimitar Berbatov added a casual third after poor Baggies defending before Rooney set up Nani to slide in late on.
With perennial rivals Chelsea, Arsenal and Liverpool all winning and United sitting in eighth prior to the late kick-off, the home side were under a degree of pressure to get a win which would really kick-start their league campaign.
They had an unusual look to their midfield with Ryan Giggs and Darren Fletcher in the centre, but this did not hamper Ronaldo, who provided a regular early outlet and looked his usual tricky self with the ball.
Rooney enjoys best week of career
West Brom, however, made a disciplined start and Chris Brunt had a decent long-range effort with United stretched at the back.
Their industrious five-man midfield formed a navy-and-white barrier with the Old Trafford crowd quiet but United had the ball in the net after 21 minutes.
Ronaldo slipped a neat pass in for Rooney who muscled in front of a defender and delayed his shot before scooping the ball past Scott Carson.
But referee Mark Halsey disallowed the effort from the England marksman, adjudging that Rooney, much to the Liverpudlian's annoyance, had pushed a Baggies man as he got to the ball.
Sixty seconds later Rooney lost his marker in the box and got his head to a whipped Giggs cross to force a decent low save from Carson.
The visitors were increasingly restricted to long-range efforts with Robert Koren firing a woeful effort into the Stretford End crowd behind Edwin van der Sar's goal after 28 minutes.
Rooney and Ronaldo soon linked up to good effect but Paul Robinson stayed admirably close to the Portuguese winger in order to nullify his attempts to get a shot off on goal.
United manager Sir Alex Ferguson was forced into making a change after 35 minutes, bringing on John O'Shea for Patrice Evra - the Frenchman seemingly having picked up a knock.
The home side continued to press but lacked any real urgency around the box, with Berbatov particularly quiet.
Rooney whipped over a dangerous corner after 40 minutes that Nemanja Vidic headed powerfully towards goal but the impressive Donk hacked away and both sides trudged down the tunnel without anything to show for their efforts.
Berbatov sprang into a semblance of life just after the break as he crossed well but Rooney could not make a proper connection in order to test Carson.
But he made no mistake when the breakthrough finally came in the 10 minutes into the second half.
Lurking on the left and with West Brom holding a high line, Rooney picked up a pass from Berbatov, cut into the box and inside Donk before firing a superb low shot inside Carson's near-post for his eighth goal in six games for club and country.
Man Utd strikers were key - Mowbray
Rooney soon had Carson pushing the ball around his post from a through-ball by Giggs, and then stung Carson's hands with a powerful strike after persistent work from O'Shea.
Baggies boss Tony Mowbray brought the hard-working but largely anonymous Roman Bednar off for Ishmael Miller to inject some fresh life into West Brom's flagging efforts just before Carson saved a Berbatov shot, which was too straight.
Ferguson sent Gary Neville on for Rafael Da Silva in the 65th minute with the young Brazilian fading after a strong first half.
And with West Brom offering very little, an inevitable second goal followed when Rooney turned provider for Ronaldo to finish easily through the legs of the over-worked Carson.
The visitors' discipline slipped further after 72 minutes when substitute Nani's cross was allowed to pass through the legs of Jonas Olsson.
It ran through to Berbatov, who stuck out a leg and poked the ball high into the net from four yards almost with disdain for his first Premier League goal for the club.
Miller went close with a decent left-foot effort with three minutes remaining and tested Van der Sar just before the final whistle.
But with a late consolation proving beyond West Brom, Nani added a fourth with a simple finish after Rooney's slide-rule cross.
West Brom manager Tony Mowbray:
"We competed ok for the vast majority of the game but the difference was the quality of their strikers.
"We did well for spells but didn't really threaten their goal. We defended naively at the end and that maybe made the scoreline a bit harsh.
"But if you have a chink in your armour, they will exploit it."
Man Utd: Van der Sar, Rafael Da Silva (Neville 65), Ferdinand, Vidic, Evra (O'Shea 36), Ronaldo, Fletcher, Giggs, Park (Nani 70), Berbatov, Rooney.
Subs Not Used: Kuszczak, Brown, Evans, Gibson.
Goals: Rooney 56, Ronaldo 69, Berbatov 71, Nani 90.
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