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Goals from Aaron Wan-Bissaka, Marcus Rashford, Bednarek and Edinson Cavani put United 4-0 up by half-time.
Martial came off the bench and scored twice, while Bruno Fernandes converted from the spot-kick and both McTominay and Dan James added their names to the scoresheet.
During the match, the 25-year-old French forward was brought down inside the box under minimal contact from Bednarek.
And many were left stunned after United were awarded a penalty and the 24-year-old Southampton centre-back was dismissed.
BT Sport has now uploaded footage of Bednarek walking down the Old Trafford tunnel and revealing what Martial told him after the incident.
The Poland international said: "It's not a foul! Martial said it's not a foul!"
Speaking after the 9-0 thumping, Ole Gunnar Solskjaer admitted that Bednarek should not have been dismissed after the foul on Martial.
"Bednarek definitely should not have been sent off," the United boss said.
Southampton boss Ralph Hasenhuttl revealed that it was "hard to explain what's going on in my mind" after United picked apart his side at Old Trafford.
"It was tough, when you lose a player after three minutes against such a team. It's tough 11 v 11," he said.
"We tried to keep a clean sheet in the second half for as long as possible. To concede nine goals again, I can take it.
"We stepped up after the first one [losing 9-0 v Leicester City at home last season] and will step up after the second one.
"The 90 minutes can be very long. The result is there and we have to live with the result again. We've had a very good season and no-one expected such an evening today.
"It doesn't reflect on what we've done this season. It's hard to explain what's going on in my mind -- we have to live it with it again."
United remain second in the Premier League and are level on points with league leaders Manchester City, though Pep Guardiola's men have two matches in hand.
Featured Image Credit: BT Sport/PA
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