An entertaining and lively start to the game saw both sides earn chances after robbing possession high upfield, while Marcus Rashford saw a strike disallowed for offside.
Goals then came through Stuart Armstrong for Saints first, then a quickfire double for United from Rashford and Anthony Martial.
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Both United men missed chances to kill the game in the second half and that proved costly as Obafemi tapped in from one yard out after Jan Bednarek flicked on a corner.
Here are the player ratings from Old Trafford on Monday night.
David de Gea – 6. Lacks a commanding presence of late. Couldn’t do much about the goal. Not often called upon from wide deliveries but a good late save from Redmond.
Aaron Wan-Bissaka – 5. Struggled on the ball when put under pressure. Decent enough going forward in spaces but caught out on the counter-attack and gave fouls away.
Victor Lindelof – 5. Solid enough in the air, not quite as assured on the ground. Lost out to Obafemi at the crucial moment.
Harry Maguire – 7. A good tussle with Ings all game long, which he emerged the victor of more often than not.
Luke Shaw – 7. Aggressive and committed with his running and his challenges. Didn’t have a lot in terms of deliveries into the box but was available as an outlet. Injured in the second half.
Nemanja Matic – 7. Some excellent passing in tight spaces and made a few important interceptions to protect the defence, too.
Paul Pogba – 7. Caught twice in possession early on – including for Saints’ goal – then started to boss play somewhat more. Not his best game overall but he made deliberate strides to get himself playing better after the poor start.
Mason Greenwood- 5. A much quieter game for the youngster, who was caught out with his defensive runs
Bruno Fernandes – 8. Always able to find space in the pockets to take the ball on the turn and find a good pass through the lines.
Marcus Rashford – 7. Scored one, had one ruled out for offside. Great run and chance in the second half to net a second and kill the game.
Anthony Martial – 8. Man of the Match. Missed a great early chance which looked costly, before leading the turnaround with an assist and a goal, both extremely well taken. Nearly scored a worldie in the last 10 minutes after taking on two.
Alex McCarthy – 7. Nothing he could do about either goal in the first half. Some good footwork to have Saints playing out. Quick off his line.
Kyle Walker-Peter – 4. Bullied and manipulated into poor positions by the pace and power of Martial and Rashford. Did manage to get forward later in the game and start delivering some dangerous crosses.
Jack Stephens – 5. Decent in defending his penalty box. Not so much nearer the halfway line. One naughty challenge on Fernandes wasn’t clever, either.
Jan Bednarek – 7. The better defender in both making clearances at important moments and also in terms of his use of the ball. Late assist for the equaliser with a great flicked-on header.
Ryan Bertrand – 7. Never gave Greenwood much of a chance to get involved in the game. Didn’t get forward as much as he’d have wanted to after the first 15 minutes.
Stuart Armstrong – 6. Great start with a few challenges to help win possession and a well-taken goal, but faded out of the game quickly.
James Ward-Prowse – 5. Caught for an early chance, then was booked. Neat on the ball afterward, but couldn’t really create much for his side.
Oriol Romeu – 5. Picked a few poor passes in the first half. Lucky not to see red for a studs challenge into Greenwood’s calf.
Nathan Redmond – 7. Hard-working, often the first outlet for the team, not afraid to challenge or run at the full-backs.
Che Adams – 5. Fruitless running and overhit passes.
Danny Ings – 7. A pain for the defence to deal with. Barely had a sniff of a chance but was good with his link-up play, his movement between the lines and his intent to get into the box.