Four second-half goals helped a fluent United see off West Bromwich Albions gallant challenge at Old Trafford.
The Reds had been frustrated by the visitors in particular keeper Scott Carson for nearly an hour but finally made the breakthrough when Wayne Rooney finished neatly for his fourth goal in as many games.
Cristiano Ronaldo made the game safe with a left-footed shot before Dimitar Berbatov prodded home and substitute Nani added gloss to the scoreline in injury-time.
Rooney was again irrepressible and the only downside to the win was an injury to Patrice Evra, who was forced off in the first half after taking a knock to his leg.
Despite the impending Champions League visit of Celtic, Sir Alex Ferguson opted to field a strong side. It was perhaps a sign of respect for the Baggies, who arrived on the back of three straight league wins.
Right-back Rafael was handed a first Premier League start, with Ryan Giggs playing in the centre of midfield, Ji-sung Park on the left and Ronaldo on the right. Carlos Tevez and Anderson were omitted from the squad after their international exertions in midweek.
West Brom began with a confidence that belied their underdog status, Chris Brunt firing straight at Edwin van der Sar when well placed on the edge of the box.
But United in particular the first three goalscorers looked in the mood as well and the forward trio were involved in most of the Reds most promising attacks.
Rooney looked to have given United the lead midway through the first half when he controlled Ronaldos pass, cut inside and dinked the ball home, only for referee Mark Halsey to harshly penalise the striker for a push on Gianni Zuiverloon.
Soon after Rooney had another chance to put United ahead but his header from Giggs left-wing free-kick was well parried by Carson.
Ten minutes before the break Evra was replaced by John OShea and the Frenchman looked to be moving uncomfortably as he left the pitch.
But United continued to press and as the half neared its conclusion Berbatov was thwarted as he prepared to pull the trigger inside the box.
The Reds began the second period in much the same vein, with Ronaldo providing extra impetus by drifting into the centre of the attack.
Rooney again looked most likely to break the deadlock but was twice denied by excellent Carson parries. Yet the Baggies keeper could do little about the strikers top-drawer finish when it finally arrived in the 56th minute.
United broke at pace, Berbatov played a ball into the path of Rooney, who charged down the left, cut inside Ryan Donk and shot low to Carsons right.
Rooney's animated celebration suggested relief as much as anything else after his earlier frustrations in front of goal. He almost grabbed a second minutes later after being played in by Giggs, only to be denied by Carson yet again.
But a second goal did arrive with 20 minutes left and this time it was Rooney who was the provider with a through-ball to Ronaldo, who shot left-footed underneath Carson.
And almost immediately Berbatov made it three with his first goal at Old Trafford. A left-wing cross from Nani, on for Park, found its way via a deflection to the Bulgarian, who instinctively stuck out a boot and guided the ball home.
West Brom battled bravely but lacked the necessary quality in front of goal. Substitute Ishmael Miller had their best chance with two minutes left but his shot whistled just wide.
And Nani made a satisfying evening for the champions even better when he converted Rooneys cross from close range in stoppage-time.
Team line-ups
MANCHESTER UNITED: Van der Sar; Rafael (Neville 65), Ferdinand, Vidic, Evra (O'Shea 36); Ronaldo, Fletcher, Giggs, Park (Nani 70); Berbatov, Rooney.
Subs not used: Brown, Evans, Gibson, Kuszczak.
WEST BROMWICH ALBION: Carson; Zuiverloon, Donk, Olsson, Robinson; Morrison, Valero, Greening, Koren (Moore 72), Brunt; Bednar (Miller 53).
Subs not used: Kiely; Cech, Hoefkens, Barnett, Macdonald.
Bookings: Zuiverloon.
Attendance: 75, 451.
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