United will have fancied their chances of getting back to winning ways against bottom of the table Southampton, but the hosts were second best from the off, deservedly falling behind to a Manuel Ugarte own goal just before halftime.
Staring down the barrel of another home defeat, United upped the ante after the break, getting back on level terms through Diallo eight minutes from time, before the Ivorian completed the turnaround in the 90th minute.
A stoppage-time third to complete his hattrick certainly flattered United, who climbed to 12th in the standings amid another disappointing season, while Southampton remain bottom with six points.
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