However, whilst Wayne Rooney's hot streak continued with a first-half brace that took him to nine goals in nine games, of far more concern to Sir Alex Ferguson was his side's shocking display after the interval, and David de Gea's goalkeeping.
De Gea carried much of the responsibility for Jay Rodriguez's early opener and looked nervy throughout, almost spilling Rickie Lambert's free-kick straight to Adam Lallana. The visitors spent almost the entire second period camped in the United half and were only denied a deserved point by some desperate defending as United failed completely to stem the tide of attacks.
United went behind after just two minutes when an underhit and misdirected back pass from Michael Carrick wasn't dealt with by De Gea and Rodriguez stroked into an empty net.
Southampton's problem was that despite their delirium, they still had another 88 minutes to get through against a side so used to coming from behind it is almost routine. They never looked likely to manage it. One excellent through-ball from Shinji Kagawa was all it took to slice through the visitors' defence, allowing Rooney to charge forward and score.
When Patrice Evra climbed highest at the far post to meet Van Persie's free-kick, the France defender left Rooney with the easiest of tap-ins from barely a yard for 2-1. With the hosts struggling to find their rhythm at the start of the second period, Mauricio Pochettino's team pushed forward in search of a leveller, with substitute Lallana having a significant influence as Lambert twice went close.
Amid the kamikaze defending that eventually heralded the introduction of Rio Ferdinand, perhaps the most telling moment came when De Gea spilled Lambert's long-range free-kick.
A gasp went round Old Trafford before De Gea pounced to deny Lallana a tap-in, sarcastic cheers - admittedly from a minority - when the keeper next caught the ball indicating the former Atletico Madrid man may be reaching the point of no return with the Red Devils.
Briefly, United relieved the pressure, with Van Persie's goalbound header brilliantly saved by Boruc before the Dutchman had an effort ruled out for a debatable offside. But Southampton were soon back on the offensive, which meant, finally, United fans were able to cheer something positive from De Gea, an excellent flying save to turn away Lambert's free-kick.