Juan Zuniga scored 53 seconds after coming on and Troy Deeney netted a stoppage-time penalty as Watford beat Manchester United 3-1 on Sunday, condemning Jose Mourinho's men to a third consecutive defeat.
A dramatic opening period saw Odion Ighalo miss an open goal for Walter Mazzarri's side, before Paul Pogba struck the crossbar from long-range for the visitors in the Premier League clash at Vicarage Road.
Etienne Capoue then put Watford ahead with his fourth goal of the season, but Marcus Rashford - handed his first Premier League start of the campaign - equalised after the break.
However, that would not prevent United from following up defeats to Manchester City and Feyenoord with another loss, Zuniga capping a great team move just moments after coming on and Deeney putting the icing on the cake from the spot.
United's latest reverse leaves them six points adrift of the early Premier League leaders, Pep Guardiola's City, and puts Mourinho under immediate pressure to turn their results around.
Watford, meanwhile, now have two consecutive victories, setting Mazzarri up superbly for a favourable run of fixtures in the weeks ahead.
Having won their first game of the season against West Ham last week, Watford started on the front foot, with Deeney inches away from tapping in Sebastian Prodl's header at the far post.
The hosts were even closer in their next attack, Ighalo guilty of a dreadful miss after Prodl's long ball caused a mix-up between United goalkeeper David de Gea and Chris Smalling. That left the Nigeria striker with an open goal, but he somehow failed to find the target.
Deeney's header was then superbly kept out by a flying De Gea, while United's first menacing attack saw Zlatan Ibrahimovic lash a good opening into the side-netting after a cross from Rashford deflected into his path.
A thrilling first half continued as Pogba collected Marouane Fellaini's pass and curled a dipping effort from 30 yards against the bar.
But the breakthrough came at the other end in the 34th minute when Martial - who had previously received a nasty knock to the head - was dispossessed by Miguel Britos in a dangerous area, with referee Michael Oliver ignoring United claims for a foul.
That allowed Daryl Janmaat to race clear down the right and cut back for Capoue to beat De Gea with a low, first-time finish and the visitors' misery was compounded by a stricken Martial having to be replaced by Ashley Young immediately after the goal.
United were initially sluggish in pursuit of an equaliser, but they struck back in the 62nd minute through their first shot on target.
Rashford was involved in the build-up - playing a one-two with Ibrahimovic - and when the Swede's cross deflected off Valon Behrami, the 18-year-old pounced to direct his shot under Heurelho Gomes from three yards out.
United came close to claiming all three points when Gomes got down impressively to claw away Ibrahimovic's powerful header from Fellaini's searching delivery, but there was a dramatic sting in the tail.
A slick Watford passing sequence saw Nordin Amrabat and Roberto Pereyra combine to set up Zuniga and he swept home on 83 minutes.
They were not done there either, with another substitute - Isaac Success - helping Zuniga to win a penalty under Fellaini's challenge in stoppage-time.
Deeney took it and powered home to delight the Vicarage Road crowd as the home side snapped a nine-game losing streak in league matches against United.