Manchester United never wins when it is expected to do so against teams which in theory are weaker than them.However against Southampton it may not be that easy to win in this weekend because they have been on a good run lately and the turn back they made against Leicester after going down one goal shows exactly that,the Saints may as well win comfortably against Manchester United if United plays like the first two games of the Premier League.If Manchester United plays very well like they did against Liverpool then most probably they are going to win this game.
I agree with you, if Manchester united can demonstrate the strategies they used to defeated Liverpool 2-1 in their next match, they will definitely win Southampton 2-1 for the favour of Manchester united. Southampton players are good in this season but I don't think they can defeat Manchester united in this match, because Manchester united defense has changed which will be difficult for their strikers to achieve a goal. no doubt, Manchester united coach will like to repeat the tactics they used to defeated Liverpool to ensure they achieve victory from this match.
After getting a win against Liverpool, of course, the mentality of the Manchester United players improved drastically because after slumping in the first 2 matches they were actually able to win against a big Liverpool team.
and for the next game I think it is very possible Manchester United will get a positive result even though against Southampton it is not easy but for a team that has high confidence and also more quality players it is very possible for Manchester United to win this game.