Southampton were a man down just two minutes into the Old Trafford clash following a horror challenge on Scott McTominay, and Ole Gunnar Solskjaer’s Red Devils took full advantage to wrap up the game by half-time.
Aaron Wan-Bissaka scored a rare goal off a Luke Shaw cross to get United up and running after 18 minutes, and Marcus Rashford doubled their advantage seven minutes later.
Jan Bednarek turned a Rashford cross into his own net after 34 minutes, and Edinson Cavani headed home another fine Shaw delivery to put United 4-0 up.
The four-goal, one-man advantage allowed Solskjaer to introduce Anthony Martial and Donny van Beek at half-time, with the pair both lacking confidence of late, with Shaw and Cavani afforded a well-earned rest.
Martial got the goal he needed with a fine finish on the turn after 69 minutes, and Scott McTominay added United’s sixth two minutes later. Jan Bednarek conceded a penalty and was sent off late on for a foul on Martial which needed a lengthy VAR review, and Bruno Fernandes finally got in on the act from the spot.
There was still time for another Martial goal, while Fernandes set up Dan James in injury time to seal a 9-0 win – the second time in Premier League history United have won by that margin, and the second time in Premier League history that the Saints have lost by that scoreline.
The huge win sends United level on points with leaders Man City and boosts their previously middling goal difference, though Pep Guardiola’s men have two games in hand over their neighbours,