Amorim then hauled off Ugarte and Hojlund, replaced by Toby Collyer and Joshua Zirkzee, as he sent a stinging message to his under-performing stars.
Sulemana raced past Yoro again and sent a shot agonisingly wide of the post.
Antony was responsible for an astonishing miss shortly before the hour mark, failing to get any purchase on a Garnacho cross.
United were now building up a head of steam despite the lingering threat from Southampton and they finally got the breakthrough in the 82nd minute.
Kyle Walker-Peters blocked Diallo’s first effort but the ball came straight back to the Ivorian, who made no mistake the second time around, blasting past Aaron Ramsdale.
But he was not finished yet.
Eight minutes later he got on the end of a cute dinked return pass from substitute Christian Eriksen before completing his hat-trick deep into stoppage time.
The win lifts United to 12th in the table and leaves Southampton 10 points from safety, looking doomed.
“We were like innocent kids in some moments and we lost the game that we controlled for 75 minutes,” Juric, still seeking his first win since arriving at Southampton last month, told the BBC.
In the other Premier League fixture on Thursday, Brighton beat relegation-threatened Ipswich 2-0, with second-half goals from Kaoru Mitoma and Georginio Rutter.
