The Saints suffered a 3-2 loss to United at St. Mary’s Stadium in the Premier League despite taking an early lead.
Graziano Pelle put Southampton ahead in the 13th minute, but United scored thrice thereafter to take a commanding 3-1 lead by the 68th minute.
Anthony Martial equalised for the Red Devils in the 34th minute and added a second five minutes after the restart. Spain international attacking midfielder Juan Mata found the target in the 68th minute.
Italian striker Pelle made it 3-2 four minutes before full time, but United held on to pick up all three points at St. Mary’s Stadium.
Following the defeat, Southampton manager Koeman took to Twitter admit his disappointment with his side’s performance.
Southampton are 16th in the Premier League table at the moment with six points from as many games, just three points above the relegation zone.