Meanwhile, Brighton's 1-1 draw with Liverpool has kept the Seagulls four points above the Premier League relegation zone.
Having gone into half time versus United with a 2-0 lead, Southampton were on course to end the weekend in third place and extend their unbeaten record to eight matches.
However, their second-half capitulation has put a different spin on matters - the Saints are now in sixth spot and having to deal with the disappointment of letting slip a sizeable advantage.
Ralph Hasenhuttl's team have gifted United a route back up the table at a time when they are struggling for consistency, but the Austrian will be more concerned with getting a positive reaction out of his own team.
While Southampton should have few issues in the final third, their backline has been far too open in recent matches, issues which started when they conceded three times during the final quarter against Aston Villa on November 1.
Although Hasenhuttl will want to win by any means necessary on Monday night, a narrow win with a clean sheet would satisfy him far more than another action-packed thriller.
The same can probably be said for Graham Potter, who has seen Brighton lose just once in six matches courtesy of getting things right in defence.
Just six goals have been shipped in as many games, and their latest showing against Liverpool suggested that Potter has finally found a better balance with his tactics.
Nevertheless, wins for West Bromwich Albion and Fulham have kept the pressure on the South Coast outfit ahead of a busy period of the season.
Danny Welbeck has starred for Brighton since his arrival at the Amex Stadium, but the Englishman requires a helping hand from his teammates when it comes to taking chances.
Brighton & Hove Albion Premier League form: DDLDWD
Southampton Premier League form: DWWWDL
Brighton will be able to call upon Tariq Lamptey, who was absent for the showdown with Liverpool through suspension.
While Danny Ings could return to the Southampton squad after a knee problem, the club's top goalscorer may be kept among the replacements.
The same can be said of Nathan Redmond, who has not featured for a month due to a hamstring injury.
Brighton & Hove Albion possible starting lineup:
Ryan; White, Dunk, Webster; Lamptey, Gross, Bissouma, March; Trossard; Connolly, Welbeck
Southampton possible starting lineup:
McCarthy; Walker-Peters, Bednarek, Vestergaard, Bertrand; Armstrong, Ward-Prowse, Romeu, Djenepo; Adams, Walcott
We say: Brighton & Hove Albion 1-2 Southampton
This has all the makings of a game which should be easy on the eye with plenty of attack-minded football combined with frailties at the back. Brighton may dominate much of the ball, but we feel that the extra cutting edge from the visitors in the final third will prove to be the difference.
Top betting tip
Watch the Sports Mole Football Shorts prediction for this game below: