Nine-man Southampton were well beaten by a Manchester United side who made them pay for an early red card that instantly handed the hosts a head start they never looked like relinquishing at Old Trafford.
Alex Jankewitz’s full Premier League debut lasted less than 90 seconds, as an overzealous challenge from the 19-year-old Swiss midfielder was rightly punished by a straight red card.
Aaron Wan-Bissaka, Marcus Rashford and Edinson Cavani were all on target in a nightmare first half, while Jan Bednarek inadvertently turned a Rashford cross into his own net.
Half-time substitute Anthony Martial made it five, while Scott McTominay, the man wounded by Jankewitz, buried a low shot from 20 yards.
Bruno Fernandes converted a contentious penalty that led to the bizarre dismissal of Bednarek four minutes from the end, leaving Saints with nine men, before Martial and Daniel James completed the scoring.
Ralph Hasenhüttl made four changes to the side that started the agonising 1-0 defeat to Aston Villa at St Mary’s, in which his team did everything but score.
With Kyle Walker-Peters added to a mounting injury list and Yan Valery loaned out to Birmingham on transfer deadline day, the right-back berth was filled by 20-year-old Kayne Ramsay, an Academy graduate who last featured for the first-team more than two years ago.
That meant James Ward-Prowse could move back into midfield alongside Jankewitz, with those positions vacated by Oriol Romeu and Ibrahima Diallo, who joined Theo Walcott in the treatment room.
Stuart Armstrong and Moussa Djenepo were the wide men, with Danny Ings partnered by Ché Adams up front.
But Hasenhüttl’s tactical plan was effectively thrown out of the window inside two minutes, when Jankewitz raised studs and caught McTominay with a costly rush of blood to the head.
Inevitably, Saints already faced a near-impossible task, but United were in ruthless mood.
Rashford twice threatened an opener; initially too unselfish in trying to tee up Bruno Fernandes, before a deflected shot was well held by Alex McCarthy.
But the breakthrough would come after 18 minutes, as Rashford rolled the ball back for Luke Shaw to cross to opposite full-back Aaron Wan-Bissaka, who converted from close range at the far post.
Rashford had been the main threat early on and the England star got himself on the scoresheet with a first-time finish from Mason Greenwood’s cutback seven minutes later.
This was the stuff of nightmares for the depleted visitors, but Ward-Prowse might have thrown Saints a lifeline after Cavani clumsily floored Djenepo just outside the penalty area.
Up stepped the skipper, who typically hit the target, but could not beat David De Gea with a vicious strike well tipped over by the keeper.
It would prove only a temporary breather for the visiting backline, as Jack Stephens dived full length to head the ball narrowly over his own crossbar from a teasing McTominay cross.
But Bednarek would be less fortunate, inadvertently turning Rashford’s driven centre just inside the far post for number three.
Cavani added his name to the scoresheet six minutes before the interval from another Shaw assist, heading home unopposed, before he thought he’d won a penalty from a lunging tackle by Ramsay.
However, VAR ruled the offence occurred outside the box, and Fernandes duly sent the resulting free-kick into the wall.
A minor reprieve, maybe, but the technology ensured Saints’ luck had not turned for long when Adams had a goal disallowed by a matter of millimetres early in the second half.
Played in by the quick-thinking Ward-Prowse at a free-kick, the striker’s first effort was blocked by Fred, before he fired a low shot across De Gea to find the bottom corner, only for VAR to intervene.
The understandably tiring Saints, with Ings operating wide on the left and Armstrong alongside Ward-Prowse in the centre, could have done with a goal to raise their spirits.
But Martial, one of two half-time substitutes for United, half-volleyed the fifth into the roof of the net from Fernandes’s lofted pass just before Ings’s legs were rested and Nathan Redmond entered the fray.
United were relentless. McCarthy parried a Greenwood shot that Stephens hooked clear but only into the path of McTominay, who drilled home from 20 yards.
Redmond’s first involvement was to make a darting run in behind the home team and play the ball square for Adams, but this time the striker could not find the net, as he blazed the chance over the bar on his left foot.
Hasenhüttl handed a debut to 19-year-old French defender Allan Tchaptchet with 12 minutes left, switching to a back three, but in truth Saints were wishing time away.
After Martial had dinked a chance narrowly wide, he was clipped in the box by Bednarek for a penalty.
Subsequent replays seemingly proved no contact, but referee Mike Dean consulted the pitch-side monitor and stuck with his on-field decision, sending off the Pole and paving the way for Fernandes to score from the spot.
Martial and James then completed the scoring on a night when everything that could go wrong did go wrong for Saints, for whom it was impossible not to sympathise.