The second half exploded into action. Tadic had a goal disallowed in front of the Stretford End for offside but that was the kick up the backside United needed to show the game still had to be put to bed. Pogba sent Martial away to fire in a shot that forced Forster into a scrambling save.
Moments later United went two up when Saints sub the faintest of touchs by Jordy Claise sent Luke Shaw tumbling in the box. Referee Anthony Taylor pointed to the spot and Ibrahimovic sent Forster the wrong way to make it 2-0.
Ibrahimovic had a chance to complete his hat-trick from a Valencia cross but got underneath the header.
Man Utd (4-2-3-1): De Gea 6; Valencia 8, Bailly 7, Blind 7, Shaw 7; Pogba 8, Fellaini 7; Mata 7 (Mkhitaryan 75, 5) Rooney 7, Martial 7 (Herrera 80,5) Ibrahimovic 8. Goals: Ibrahimovic 36, 52pen.
Southampton (4-2-3-1): Forster 6; Cedric 6, Fonte 6, Van Dijk 6, Targett 6; Romeu 5 (Clasie 11, 5); Davis 6 (Austin 66, 5); Tadic 7, Hojbjerg 6, Redmond 6; Long 6 (Rodriguez 84).