Jan Bednarek and James Ward-Prowse both struck from dead ball situations to put Ralph Hasenhuttl's men in a strong position, but super-sub Edinson Cavani's flipped the scoreline on its head with two goals and an assist to send United back into the top half.
United were able to get into an attacking rhythm, piecing together some quick passes and intricate passages of play that Southampton simply couldn't deal with. When they kept it simple and moved it across the deck as opposed to going the aerial route they were also more successful. There's an argument to be made that their cardio played a role in their comeback here.
United struggled when it came to the hosts' physicality and defensively had problems dealing with Southampton's direct service in the final third. Set pieces continue to be one of their most obvious weak spots.
Manager rating out of 10
7 -- Ole Gunnar Solskjaer looked like a shell of a man towards the end of the first half and rightly so, as it just seemed as if he didn't have a Plan B. The zonal marking that he refuses to drop cost the Red Devils in a big way for the first goal, but thankfully for him, the changes he made at half-time, namely Cavani, proved a masterstroke and were integral to his side fighting their way back into the game.
Player ratings (1-10; 10 = best. Players introduced after 70 minutes get no rating)
GK David de Gea, 4 -- The Spaniard had a nightmare of a day at the office as he failed to control his area for the opener, should've done better with Ward-Prowse's free-kick for Saints' second, and had to come off through injury at half-time.
DF Victor Lindelof, 7 -- The centre-back didn't really link up with Wan-Bissaka down the right in the way he would've hoped, but he was effective enough in keeping the Saints at bay.
DF Harry Maguire, 6 -- While he may have made a handful of important interceptions, Maguire's negative play and constant slowing down of the tempo does more harm than good.
DF Alex Telles, 7 -- Produced some wonderful link-up play but still needs to find the balance between surging forward and keeping things tight down the flank.
MF Fred, 7 -- Gave away the free-kick that allowed Ward-Prowse to make it 2-0, but after a sloppy first half, he improved dramatically when he pushed further up. Featured in two of United's three second-half goals.
MF Nemanja Matic, 5 -- From poor passing to a lack of control in the middle of the park, this was a day to forget for Matic.
MF Donny van de Beek, 5 -- Needs to learn how to pull the trigger after a series of missed opportunities and hesitation on the ball.
MF Bruno Fernandes, 8 -- After a lethargic first 45 minutes, Fernandes burst into life with a nice finish for United's first and a fortunate shot-turned-cross for Cavani's equalizer.
FW Marcus Rashford, 6 -- Lost his man for the opening goal and should've done better when he raced through in the 52nd minute. His persistence paid off, though, with a lovely delivery for Cavani's winner.
FW Mason Greenwood, 5 -- Had to drop far too deep in order to get any sort of service. His best chance came in the 30th minute when he shot straight at the goalkeeper.
GK Dean Henderson, 7 -- Made some controlled saves at the start of the second half and more than held his own after replacing the injured De Gea.
DF Brandon Williams, N/R -- Was seen as an odd entry into the game and didn't really make much of an impact in the last 10 minutes.
FW Edinson Cavani, 9 -- The former PSG star was a breath of fresh air and caused Southampton plenty of problems, eventually leading to him assisting one goal and scoring two more with great, instinctive headers.