Rooney took his seasonal tally to 26 with a goal either side of the interval as United took advantage of Manchester City's shock defeat at Swansea to assume that familiar position at the head of affairs on the title run-in.
And with United at relegation-haunted Wolves next Sunday, City could find themselves four points adrift before they take to the field again, against resurgent Chelsea, at the Etihad Stadium on March 21.
That it could turn out to be such a pivotal day should not come as a surprise.
Since United won at Tottenham a week ago, the battle to be champions has been condensed into who wins the derby on April 30, with the eight games in between largely an irrelevance.
However, the traps that have to be negotiated on the path to glory are not insignificant as United, even with all their experience, so nearly found out and City did.
The Old Trafford outfit were still trying to adjust to a line-up with only one conventional winger in Ashley Young when Keith Andrews slid in to reach Liam Ridgewell's low left-wing cross.
It seemed the Irishman was sure to score. Instead, he prodded his effort wide.
Then, when Chris Brunt had touched a 30-yard free-kick into his path, Andrew let fly with a thunderous shot that would have found the bottom corner had David de Gea, not for the first time in recent games, not made a fine, full-length save.
It was the cue for Sir Alex Ferguson's side to awake from their slumber and Young to have an impact.
If Nani is going to be out for a few weeks as pre-match rumours suggested, Young is going to be a key man and needs to find the form he produced in his first few weeks as a United player.
Together with skipper Patrice Evra, Young tormented the right side of West Brom's defence.
The former Aston Villa man had a shot beaten away by Ben Foster, then ran on to Danny Welbeck's back-heeled through ball before cutting a cute cross back into the box which Jonas Olsson managed to scramble away.
When United's opener came, it was a wonder Young was not involved.
The Premier League could have been forgiven a sharp intake of breath as Foster immediately appealed for offside as Rooney touched home Javier Hernandez's cross from what did appear to be an illegal position.
After what went on at Bolton yesterday, further controversy was the last thing they needed.
Happily, in this instance, TV replays showed the officials had got it absolutely spot on as Rooney timed his run to perfection before netting for the fourth time in a week.
Peter Odemwingie had been a subdued presence during the opening period.
But when he went over Evra's half-hearted challenge, he was convinced West Brom should have had a penalty. He might have been right too, as Evra did not play the ball and dangled a leg out for the striker to fall on.
It was another of those fortunate moments United make a habit of capitalising on.
This time, they could not do it as Hernandez drove a shot against a post, Rooney's overhead kick went straight to Foster and, most startlingly of all, Welbeck slid his shot wide of an open goal after the entire West Brom defence, including their goalkeeper, had got caught out by Scholes' inspired long pass.
Booked at the start of the second period for a foul on Rooney, Olsson brought his afternoon to an early conclusion when he was beaten to Welbeck's pass by Hernandez and chopped the Mexican down instead.
Within five minutes, the hosts had their second, Rooney again the scorer, slotting home a clinical penalty after Andrews had fouled Young.
Among the plethora of substitutions that followed came Paul Pogba's for Scholes, which tends to shoot down suggestions that the teenage Frenchman has agreed to join Juventus in the summer.
Young and Hernandez both wasted chances to reduce the goal difference advantage City have in their favour at present.
However, it may not come down to that. And at Molineux next Sunday, it will hardly please under-fire Wolves boss Terry Connor to know United could really turn the screw.