Sheriff managed to win the first match. But they were defeated in the next three matches. And they are 6 points behind United, with only 3 points. I think Manchester United will qualify for the next round from the top of the points table and Real Sociedad will qualify for the playoffs. However, Sociedad is in very good form in the Europa League. Manchester United must win against Sociedad if they want to go top of the table.
I also have an opinion that is not much different from yours, where if Manchester United is able to beat Real Sociedad in the Europa League group, then Manchester United will win in group E of the Europa League this season and that will clearly ensure that he advances to the next round.
And if you look at last year's results which were also in the Europa League, Manchester United had won a lot at home to Real Sociedad, even though it was a draw when playing at home (Old Trafford). So it is still very possible for Manchester United to win against Real Sociedad in the final match for group E. Even then, if the Manchester United players can be quite consistent with their performances in the match.