Southampton is not an easy opponent, especially at this time man united is not in a good form, the result of the match may be won by southampton if man united again play badly like in the first two games.
However, if Casemiro can start his first match, I think Man United will have more control over the match against Southampton, and maybe Man United can maximize every opportunity and make it a win.
As for Southampton this season for 3 matches, they are slightly better than Manchester United but this does not mean they are the favorites because United are definitely superior but when you see Ten Hag's scheme against Liverpool I might expect a lot more with Manchester United in this match.