It was a poor performance from start to finish for the hosts, who had Romelu Lukaku stretchered off early on with a head injury, while Paul Pogba was guilty of costing United a goal after he diverted an already goal-bound effort in from an offside position.
The result left the Red Devils 14 points behind league leaders Manchester City—who have a game in hand—and their insipid play in front of goal was reminiscent of last season's struggles, which included just 54 goals and 15 draws.
Lukaku should have done better with a header in the opening minutes when he rose to meet Juan Mata's cross, but he had no opportunity to make amends for the miss as he was taken off shortly after clashing heads with Wesley Hoedt, per Indy Football:
The Belgian had failed to hit the target from six yards, and Hoedt did the same with a header of his own, but Southampton tested David De Gea through James Ward-Prowse, with the goalkeeper having to make a fine stop down to his left.
At the other end, Alex McCarthy was on hand to deny Mata when the ball fell to him inside the area, while Marcus Rashford and Jesse Lingard couldn't convert fine crosses from Ashley Young and Henrikh Mkhitaryan. Their overall play, however, lacked verve and ingenuity.
United could have also considered themselves unfortunate not to have had a penalty after Maya Yoshida handled in the area.
Per James Robson of the Manchester Evening News, manager Jose Mourinho blamed the result on the decision, though Samuel Luckhurst of the same outlet believed that was the wrong approach:
The visitors almost broke the deadlock early in the second half when Shane Long fired a first-time effort at De Gea, but the Spaniard reacted quickly to divert the low shot over the bar with his feet.
Saints perhaps looked the more threatening outfit but didn't create further clear-cut opportunities.
The hosts were even less inspiring in the second half, with opportunities few and far between and fluency at a premium, per ESPN FC's Rob Dawson:
They did have the ball in the net, however, when Pogba turned in Nemanja Matic's effort from close range only for it to be correctly disallowed for offside—though replays indicated the ball would have gone in without the Frenchman's intervention.
The Red Devils travel to Goodison Park on Monday to face Everton.