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.
Southampton is not really bigger team than Liverpool and fact should be easy for Manchester United to win for next match, but still problem with inconsistent performance usually faced by United. Still depending who will be settle as regular position for next match when Manchester United against with Southampton, I think keep make Cristiano Ronaldo and Harry Maguire on the bench still give opportunity for winning than have change position last match success defeating Liverpool. But Manchester United have another great player and actually Casemiro will settle one place on holding midfielder for Manchester United as regular player against with Southampton, who will begin match on the bench if Casemiro as regular player?