INJURY-RAVAGED Albion were sentenced to a fourth successive defeat by sub Javier Hernandez's 75th-minute header - but that only told half the story.
The Baggies fell behind to Wayne Rooney's third-minute header but responded magnificently and deservedly equalised through James Morrison's stunning 14th-minute volley.
Roberto Di Matteo's men then took control and were denied a clear-cut penalty by referee Chris Foy when Graham Dorrans was fouled in the box by Gary Neville in the 26th minute.
Albion DID win a penalty in the 63rd minute - but top-scorer Peter Odemwingie wastefully fired his spot-kick wide.
The Baggies suffered further blows as defenders Steven Reid and Pablo Ibanez hobbled off in quick succession - and United took full advantage when Hernandez headed home from Rooney's corner.
It was tough on the hosts who were the better team for long spells and would have been dreaming of beating United for the first time in 27 years had Odemwingie converted his penalty.
Reid was handed his first Barclays Premier League start in seven games as Di Matteo made four changes - two enforced.
The ex-Republic of Ireland international was joined by Ibanez, the back-from-suspension Chris Brunt and Marek Cech in earning recalls.
The men to make way were suspended duo Gonzalo Jara and Gabriel Tamas plus Somen Tchoyi and Nicky Shorey.
Sir Alex Ferguson also made four changes from the 1-1 draw at Blues on Tuesday.
Ex-Baggies keeper Tomasz Kuszczak was given a warm reception as he started between the sticks in place of Edwin Van der Sar. Neville, Darren Fletcher and Gabriel Obertan also came in for Darron Gibson, Rafael and Ryan Giggs.
Albion started on the front foot and Morrison tried his luck from 30 yards after only 18 seconds - but Kuszczak was equal to it.
However, the Baggies' makeshift defence was found wanting with little over two minutes on the clock as United scored from their first attack.
Obertan pulled the ball back to Patrice Evra who found the unmarked Rooney with a first-time cross and the England striker had the simple of task of heading past Scott Carson from 12 yards.
It was an early hammer blow to a team that had lost their last three games, but to their credit, Albion regrouped and began to peg United back.
On 7, Morrison whipped in a terrific right-wing cross and Jerome Thomas got in front of the dozing Neville, only for his first touch to let him down and the chance was gone.
Morrison had enjoyed a lively start to the game and clawed Albion level with a goal-of-the-season contender on 14.
Under pressure from Odemwingie, Nemanja Vidic could only head Brunt's diagonal ball in the direction of Morrison and the Scotland international beat Kuszczak all ends up with a stunning 25-yard volley which curled into the far top corner.
The Baggies looked a transformed side now and after great build-up play, Cech fired an awkward chance well wide from the edge of the box.
Kuszczak then had to take evasive action to prevent Thomas getting on the end of Morrison's diagonal ball into the box.
It was all Albion and Thomas spearheaded another fine Baggies break on 23, only to shoot tamely at Kuszczak from 20 yards.
Then, on 26, a major talking point.
Dorrans looked certain to score as he chested down Morrison's defence-splitting pass inside the box, but just as he shaped to shoot, he appeared to be tripped by Neville.
However, referee Foy, who awarded Stoke two debateable penalties here in November, incredibly waved play on.
Albion immediately regained possession and Brunt advanced forward before firing inches over from 25 yards.
United's opener had come from their only effort at goal thus far and the visitors were struggling to get anywhere near Carson's net as the half drew to a close.
In fact, it was Albion who threatened again on 39 when Odemwingie failed to match Albion's neat approach play by firing well wide from 20 yards.
A great chance went begging in the first minute of stoppage time when Odemwingie picked out Dorrans' run. The Scotland international advanced into the box before drilling left-footed into Kuszczak's side-netting from 15 yards.
The Baggies had out-classed the league leaders since going behind and they went close again when Brunt's free-kick clipped the outside of the United wall and flew wide.
Albion were straight back on the attack after the break and Thomas surged down the left before teeing up Brunt, who flashed a shot wide from 18 yards. That made it 9-1 to Albion in shots at goal.
Yet another Baggies attack resulted in Dorrans seeing his shot from the edge of the box deflected wide by Anderson.
From the resulting corner, Paul Scharner went even closer when he flashed a diving header just wide from Brunt's centre.
The Red Devils were so second best that Ferguson rang two changes just past the hour mark as Hernandez and Gibson came on for Dimitar Berbatov and Obertan.
Then, on 63, Albion finally got the chance their overall play had deserved to take the lead when Foy pointed to the spot after Rio Ferdinand had tripped Thomas inside the box.
However, Odemwingie got his penalty horribly wrong, scuffing the ball harmlessly wide from 12 yards.
The miss breathed new life into United and Carson had to produce a brave save at the feet of Rooney after the former Everton ace had charged through the Baggies' defence.
Albion's task became even harder in the 71st minute when Ibanez hobbled off and was replaced by Tchoyi. It came on the back of the injured Reid being replaced by Gianni Zuiverloon, recalled last week from a loan spell at Ipswich, seven minutes earlier.
United were now enjoying their most dominant spell of the game and Carson did well to keep out Michael Carrick's angled drive from just inside the box.
However, it proved a momentary reprieve. From Rooney's ensuing corner, sub Hernandez was left totally unmarked and headed home from inside the six-yard box.
United moved in for the kill but Rooney could only head over from Fletcher's centre.
But Albion weren't finished yet and Thomas went mightily close when he cut inside sub Fabio before seeing his goalbound shot blocked by Vidic.
Brunt then sent a blistering 25-yard drive just over before that man Thomas exchanged a neat one-two with Dorrans and missed the target from a tight angle.
With Albion pressing for an equaliser, it was inevitable that United would catch them on the break and the hosts were grateful for Zuiverloon's last-ditch challenge, which denied Hernandez a clear sight of goal.
The Baggies kept pressing until the final seconds and Morrison wasn't far away with another long-range effort in the fifth minute of stoppage time.
But Foy's final whistle signalled another hard-luck story in which Albion did not get their just desserts.
ALBION (4-2-3-1): Carson; S Reid (Zuiverloon 64), Scharner, Ibanez (Tchoyi 71), Cech; Mulumbu (Fortuné 90+4), Dorrans; Morrison, Brunt, Thomas; Odemwingie. Subs not used: Myhill (gk), Miller, Shorey, Cox.
UNITED (4-4-2): Kuszczak; Neville (Fabio 71), Ferdinand, Vidic, Evra; Obertan (Gibson 61), Fletcher, Carrick, Anderson; Berbatov (Hernandez 61), Rooney. Subs not used: Amos (gk), Owen, Evans, Bebe.
GOALS: ALBION - Morrison (14). UNITED - Rooney (3), Hernandez (75).
BOOKINGS: ALBION - Brunt (foul 88). UNITED - Hernandez (foul 83), Fabio (foul 84), Vidic (foul 90+2).
REFEREE: C Foy (Merseyside).
ALBION STARMAN: Chris Brunt.
PIC: GOAL GLEE...Morrison celebrates his stunning equaliser