West Brom earned their first home league win against Manchester United for 32 years as Salomon Rondon's low strike beat the 10-man visitors.
United played for more than an hour without Juan Mata, who received two bookings inside three minutes.
But Albion created little until Rondon found space to fire in Sebastien Pocognoli's dangerous left-wing cross.
United rarely looked like equalising as they lost ground on the top four.
The Red Devils are sixth in the Premier League table, failing to move back above West Ham and remaining three points adrift of fourth-placed Manchester City.
Victory moved Albion up to 11th, their tally of 39 points almost certainly securing top-flight survival for a sixth successive season.
Manchester United looked assured in the opening 25 minutes as they dominated possession against the Baggies, albeit without really troubling home keeper Ben Foster.
But the entire complexion of the match turned on the stupidity of the normally-intelligent Mata.
The Spain international was shown his first yellow card for rashly blocking Darren Fletcher's quick free-kick, then he was given a second when he needlessly clipped the heels of the former United midfielder within three minutes.
Within seconds of Mata's dismissal, Albion almost took the lead when Craig Dawson's ambitious 25-yard punt looped over David De Gea but dropped just past the wrong side of the left-hand post.
An Albion onslaught failed to materialise in a dour end to the first half, while United almost opened the scoring through Marcus Rashford's instinctive drilled shot and Jesse Lingard's dipping angled drive before the hour mark.
But the home side gradually grew in stature as United tired, finding the breakthrough through Rondon's smart finish.
"I thought we looked flat when they went to 10 men but the advantage of 11 v 10 seemed to get us through in the end," said Scotland midfielder Fletcher afterwards.
Many Albion supporters have not warmed to Tony Pulis since his arrival in January 2015, complaining about the style of football the ex-Stoke manager has brought to The Hawthorns.
But what they cannot dispute is the fact the Welshman has steered them away from a relegation scrap for the second successive season.
Pulis faced heavy criticism last month after a dire 1-0 defeat at Newcastle left them six points above the drop-zone and without a win in five games.
However, their fortunes have dramatically improved and they have now taken 10 points - three wins and a draw - from their past four matches.
In addition to almost securing survival, Pulis has also achieved something that no Albion manager has since March 1984 - a home win against the Red Devils.
Once the home side went ahead - with their only on-target shot of the second half - they did not look like relinquishing their slender lead.
United switched to a 4-4-1 formation after Mata's dismissal, leaving French youngster Martial as an isolated lone striker.
Despite a brief spell where United threatened through Martial and Lingard, the visitors failed to force Albion keeper Foster into a single save.
The Baggies are back in Premier League action on Saturday, travelling to title-chasing Arsenal knowing that a win could move them into the top half.
Meanwhile, Manchester United's attention switches to the Europa League and the home leg of their last-16 tie against Liverpool.
With BBC
