The victory came after back-to-back losses for Manchester United
Matthijs de Ligt scored his first goal for Manchester United as Erik ten Hag's side claimed a much-needed Premier League victory at Southampton.
The Red Devils went into the recent international break on a low after being comprehensively beaten by Liverpool, and needed to put in a performance on their return to action.
They survived a difficult start before a penalty save by Andre Onana from Cameron Archer proved the turning point as soon after they took the lead.
Summer signing De Ligt got the opener, heading into the bottom corner from Bruno Fernandes' fantastic ball into the box.
Six minutes later Manchester United doubled their lead, Marcus Rashford opening his account for the season with his first goal since March, guiding a low strike through a crowd into the net.
The double blow seemed to quell Southampton's eagerness and Manchester United were comfortable in the second half.
A bad day was rounded off for Southampton when Jack Stephens was shown a straight red card late on for a reckless challenge on Alejandro Garnacho.
The substitute then wrapped up the win for the visitors with a thumping finish into the roof of the net deep into stoppage time.
Much-needed win for Red Devils
Southampton 0-3 Manchester United: Erik ten Hag reaction
It did not take long for the joy of the FA Cup final win against rivals Manchester City to fade, with the 3-0 loss at home to Liverpool bringing manager Ten Hag into the spotlight once again.
Casemiro endured a difficult game against Liverpool and paid the price by being dropped to the bench at Southampton.
It initially did not look like the international break had provided the reset Manchester United fans will have hoped for as they were second best for the first half hour.
But Southampton could not take their chances, most notably when Diogo Dalot brought down Tyler Dibling inside the box. Archer stepped up to take the penalty but his strike was a poor one and kept out by Onana.
From then on Manchester United were in full control, and while there is still clearly work to be done to iron out some issues in the team, three goals and a clean sheet is a positive return to action for Ten Hag's side.
Saints pay price for profligacy
Southampton 0-3 Manchester United: Russell Martin reaction
It has been a miserable return to the Premier League for Southampton as defeat leaves them waiting for their first points of the campaign.
Manager Russell Martin is sticking steadfastly to the attractive style of football that got them promoted from the Championship, and they should have gained some reward for their excellent start to this game.
The hosts produced some slick build-up play but that was undone by their inability to make it count in the final third and, in the Premier League, failure to be clinical can swiftly be punished, as it proved.
One notable bright spot for Southampton was the performance of Dibling. The 18-year-old midfielder made his debut and was a menace to the Manchester United defence in the first half.
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