By Nathan Youd.
United dominated for long phases of the game but had to defend for a nervy last few minutes as the visitors rallied to try and steal a point.
United settled early and bossed the early stages of the game with the visitors looking content to sit back and catch United on the break.
Ashley Fletcher perhaps could have done better after finding space in the box after 20 minutes, but the forward couldn’t get the ball out of his feet.
Fletcher would make amends just 5 minutes later to put United in front. Harrop reacted well to a botched punch-clear by Isted to curl the ball onto the post from 18 yards, Joe Riley met the loose ball and fired against the Southampton defender on the line but Fletcher showed a good poachers instinct to find the back of the net.
United almost made it two soon after, but Riley fired straight at Isted after being released by a brilliant through-ball by the impressive Willock.
United had to remain careful to watch the visitors on the counter attack as Sam Gallagher headed wide with a smart header.
The young reds doubled their advantage moments later and they were indebted to a fantastic run from Joshua Harrop. The midfielder exchanged passes with Riley and charged through the Southampton defence to slide a pass to Ashley Fletcher who made no mistake from six yards to score his and United’s second.
Sam Gallagher gave United a scare soon after. The striker controlled an angled delivery but could only drag his shot wide of the goal.
That proved to be the last action of the first half as United entered the break two goals to the good and looking good value for their lead.
Sean Goss replaced Callum Evans at half time for United as the reds looked to pick up where they left off.
The substitute almost added a third for United on the hour mark but he was denied by a smart double-save from Isted.
Straight down the other end, Sam Gallagher wasted another chance for the visitors, flashing wide when he perhaps could have played in Hesketh.
Fletcher was denied a hat trick by another good Isted save after a great delivery from Harrop, who caused Saints problems throughout the game.
The visitors were struggling to get out of their own half as United pushed forward to find the third goal which would kill the game off.
But against the run of play Saints stunned United with the next goal to reduce the deficit. Substitute Marcus Barnes skipped past Willock and fired a 25-yard screamer into the bottom corner past the despairing dive of Sutherland.
Barnes almost got Southampton level on 73 minutes as he flashed wide to start the alarm bells ringing for United as Saints began to look the most dangerous team.
Gallagher could only guide a Barnes cross wide five minutes later as Saints missed another guilt edged chance to level the scores.
The game was teetering on a knife-edge with both sides going at each other end to end. Ashley Fletcher wasted another chance of a hat trick to put the game to bed as he drilled wide in the box.
Barnes had the last Saints chance of the game but blazed wide from 18 yards when he should have at least tested Sutherland.
United held on through the last stages of the game to record a valuable victory to consolidate their position at the top of the table.
Man of the Match: Josh Harrop
Manchester United Starting XI: Henderson; Evans (Goss 45′), McConnell, Borthwick-Jackson, Barber; Riley, Harrop, Willock, Mitchell; Croskery (Redmond 60′), Fletcher.
Unused substitutes: Thompson, Pereira.
Southampton Starting XI: Isted; Leggett, Clinton, Deabayo, Nehemie; Higgins, Mason; Demkiv, Hesketh, Sims; Gallagher.