After ending 2023/24 on a high, it has been a difficult start to this campaign for the Red Devils after back-to-back PL defeats.
As such, Erik Ten Hag could ill afford another poor result on the south coast, especially against a Southampton outfit without a point to their name since their return to the top flight.
Despite losing each of their first three league games, the Saints averaged 68% possession across those encounters and enjoyed much more of the ball early on here.
On his first start for the club, 18-year-old Tyler Dibling was particularly dangerous, drawing an important save from Andre Onana inside the opening 10 minutes.
United did threaten, with Joshua Zirkzee and Noussair Mazraoui drawing stops from Aaron Ramsdale, but it was the hosts who had the first clear chance to break the deadlock.
Dibling was again the architect, beating Diogo Dalot down the right flank and drawing a foul from the Portuguese in the area. However, from the subsequent penalty, Onana denied Cameron Archer before holding the rebound.
That ultimately proved to be a turning point in the encounter - Ramsdale got down fantastically to tip Zirkzee’s low drive behind for a corner, but there was nothing he could do to keep out summer signing Matthijs De Ligt’s header shortly after - his first goal for the club.
Then, shortly before half time, Marcus Rashford ended a run of nine games without a PL goal as he curled an effort into the far corner via the post.
The strong end to the first period gave United confidence, and they were far more controlled after the interval.
Rashford, buoyed by his earlier strike, caused Southampton more problems, with Ramsdale tipping a dipping shot from distance over the bar.
Russell Martin’s outfit continued to lack cutting edge, and although they did attempt to put their opponents under pressure, they were never able to for a sustained period.
At one point, this looked like it would be a difficult day at the office for the visitors, but it actually proved to be serene as they strolled to all three points - a task made even simpler by Jack Stephens’ late red card for a high challenge on Alejandro Garnacho.
The Argentina international rubbed salt in the Saints' wounds deep into stoppage time, rifling home from Dalot's comeback to confirm a win that doubles United's points tally for the campaign and means they are unbeaten in their last 16 head-to-head, while Southampton suffered a fourth successive defeat to leave them rooted inside the relegation zone at this early stage.
Flashscore Man of the Match: Bruno Fernandes (Manchester United)