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It's quite reasonable that Real Sociedad is more favored than Manchester United because Real Sociedad will have the advantage of playing at home, and as we know, Man United has inconsistent performance. But still, that doesn't rule out the fact that Man United could still make a surprise, because even though they are in ruins in the league, but what is happening in the Europa League is the opposite, where Man United can perform well and they get to the last 16 without having to go through the play-offs.
Real Sociedad have the advantage of playing at home, but it should be underlined that Man United qualified for the last 16 after finishing third, while Sociedad had to go through a play-off. Real Sociedad should not underestimate Man United even though their performance in the domestic league is very disappointing, Amorim's team has been able to perform well in the UEL with an unbeaten record in 8 Ligue Stage matches and has managed to achieve consecutive wins in the last 5 games. I think Man United has a big chance to give a surprise in this game because Sociedad record in European competitions is not very good.
Manchester United's inconsistent performance makes Sociedad the favorite. As you said, they will also use the advantage of playing at their home ground. If Sociedad starts the match well and scores an early goal, then things can go their way. United is a team of quality players individually, but they are not good as a team and have not been able to show the performance they want in many matches. If Sociedad can use this advantage, they can finish the match with the scores they want.
I don't think it will be a match with many goals because the teams will think about defense first. If an early goal is scored, maybe the situation can change.