He sealed the remarkable comeback with a diving header three minutes from time as Alex Ferguson's men yet again plucked victory from the jaws of the defeat thanks to their incessant pressure and clinical finishing.
"The only thing in my mind is to do my best and do what the gaffer told me to do. He told me to do the things I do in training, to run in behind, to move the defenders," Hernandez told Sky before claiming a hat-trick despite a debatable second.
"Yes of course (I'll claim it). I hit the target and the defender touched the ball."
United stretched their lead to four points after 11 games ahead of second-placed Chelsea with champions Manchester City a further point back in third. Chelsea host Liverpool and City entertain Tottenham Hotspur on Sunday.
Arsenal's season took another turn for the worse when they let slip a two-goal lead and Mikel Arteta missed a late penalty as a Dimitar Berbatov-inspired Fulham grabbed a 3-3 away draw.
Olivier Giroud's double - his first home league goals - could not prevent Arsenal staying seventh, sliding four points behind fourth-placed Everton who beat visiting Sunderland 2-1.
Surprise packages West Bromwich Albion, in fifth, are level on points with Everton following a 2-1 victory at Wigan Athletic thanks to James Morrison's header and a Gary Caldwell own goal.
Former Liverpool midfielder Charlie Adam's first goal for Stoke City sealed a 1-0 home win over Queens Park Rangers who plunged to the bottom of the table after Southampton leapfrogged the Londoners with a 1-1 draw against Swansea City.
Reading and Norwich City drew 0-0 in a dour game.
Proceedings at the late game at Villa Park were far from bland with 21-year-old Weimann making United pay for a lacklustre first half when he slammed in from the edge of the area to beat goalkeeper David de Gea above his head.
He slotted in a second five minutes after the restart to further upset the form book with Villa languishing in 17th spot.
Memories of the long ago days when they would often compete with United started to drift back into the minds of the Villa faithful but the sight of Hernandez ready to come on at the start of the second period should have rung alarm bells.
The arch goalpoacher latched on to Paul Scholes' lobbed pass to score and was soon celebrating again even if replays suggested his expert finish after a good move might have gone wide without hitting Vlaar.
Robin van Persie struck the woodwork twice in succession as United poured forward for the winner and the Dutchman supplied the cross from a free kick for Hernandez to head home.
Arsenal have stuttered all season but got off to the perfect start against Fulham when close-season signing Giroud finally began to show some of the might which helped him win the French title last term with Montpellier.
The tall striker netted with a powerful header on 11 minutes after a corner from the recalled Theo Walcott and the hosts were soon 2-0 up and cruising when Arteta set up Germany's Lukas Podolski for an easy finish midway through the first half.
The unmarked Berbatov pulled one back on the half hour when he headed in a corner and the laidback Bulgarian set up substitute Alex Kacaniklic as Fulham equalised five minutes before the break.
Berbatov put Fulham 3-2 when he converted the coolest of penalties after a foul by Arteta in the 67th minute.
Giroud then took another small step in his quest to replace ex-Arsenal talisman Van Persie by netting another header before Mark Schwarzer saved Arteta's stoppage-time spot kick.
Everton have been in great form but looked to be heading for defeat against Sunderland when Adam Johnson netted on the stroke of halftime to become the first player other than Steven Fletcher to score for the visitors in the league this term.
No one has epitomised Everton's drive this season more than Belgium midfielder Marouane Fellaini and he fittingly grabbed the equaliser by drilling the ball home on 76 minutes.
Three minutes later Fellaini played in Nikica Jelavic for the winner in manager David Moyes's 400th Premier League match.