Last year, Leicester City humiliated Southampton by the same scoreline and this time it was a rampant United, matching the Premier League’s biggest margin of victory thanks to four first-half goals and five in the last 21 minutes.
Ralph Hasenhüttl’s Southampton also picked up two red cards — one for teenager Alex Jankewitz inside the first 90 seconds of his full Premier League debut and a second when Jan Bednarek clipped Anthony Martial in the box.
Martial, Aaron Wan-Bissaka, Marcus Rashford, Edinson Cavani, Scott McTominay, Bruno Fernandes and Daniel James were all on a packed scoresheet for United, while Bednarek also conceded an own-goal in the first half.
“We tried to defend, but it’s clear when you have 90 minutes here to work against the ball it’s difficult.
“We have this experience before in the Premier League, conceding nine goals. It was again nine, what can I say? It’s very disappointing and frustrating but we’ve stood up once, we’ll stand up again.”
After Jankewitz’s early dismissal for a lunge on McTominay, Wan-Bissaka opened the scoring when he got on the end of Luke Shaw’s cross.
Rashford then doubled United’s lead before seeing Bednarek turn his cross into the Southampton net. A Cavani header then made it 4-0 at the break.
Saints held on for much of the second half but Martial broke their resistance by firing past Alex McCarthy after a deft piece of control in the box.
McTominay scored United’s sixth two minutes later when he rifled in a low strike from outside the area.
The seventh goal came after a video assistant referee review deemed that Bednarek had brought down Martial in the box with Mike Dean showing the defender a red card despite what appeared to be minimal contact.
Fernandes scored the penalty before Martial got his second and James completed the scoring in extra time.
“We used the width, we had movements in behind, we had strikers who wanted to get on the end of it scoring goals, even had full-backs wanting to be on the end of it.
“It was one of those performances you once in a while get and you enjoy. I am sure everyone enjoyed being out there in a Man United shirt.”
The other occasion that a team has won by a nine-goal margin in the Premier League was United’s victory over Ipswich Town in 1995.
Two teams received two red cards on Tuesday as Arsenal also saw David Luiz and Bernd Leno sent off in a 2-1 loss to Wolves.
Bottom side Sheffield United recorded its third win of the season with a 2-1 victory over Premier League rival West Brom, while Crystal Palace defeated Newcastle by the same scoreline.