Currently Southampton got defeated 3 of 3 matches in Premier League and become bad performance as promotion teams, I don't think difficult for Manchester United to secure three points although their performance still inconsistent after two consecutives loss from Brighton Albion and Liverpool but they have good composition players than Southampton not winning one match yet.
Manchester United have many busy upcoming matches later after facing Southampton four days later will play on EFL Cup third round.
Its important tactician from Erik ten Hag how to manage their players without injury on the most busy schedule matches, Manchester United must return back to the winning trend if won't loss opportunity with top fourth standings position and have finish outside from Champion League spot standings two consecutives season.
I think Manchester United will probably beat Southampton. Southampton has lost all their Premier League games so far which is not good for new team in league. Manchester United has better players but they have a lot of games coming up soon. They need to be careful not to get any players hurt. After they play Southampton they have another game just four days later. So manager Erik ten Hag needs to make smart decisions to keep his players safe. Manchester United really needs to start winning again or they might not get to play in Champions League for third year in row.