Manchester United's league performance has been really bad, so it's only normal that they've received some backlash from the fans. Many people were very surprised that they finished so poorly in the league. But the fact that they were able to win the FA Cup is a big advantage for them. In this way, they won at least one championship. Also, it was not easy for them to beat a team like Manchester City.
Maybe next season they can start the league better and finish the league with a better ranking
In fact, supporters have been protesting strongly against Manchester United for a long time, and some time ago they even held a demonstration to sell their ownership of Manchester United because they thought that their owners were not serious about managing the club and only prioritized business rather than improving the club's performance.
I can understand their feelings, because as supporters of course they always want to see their proud club at a good level to continue playing in the competition. They are one of the big clubs in the Premier League or even Europe, so what they have shown lately is far from good and it deviates from their status as one of the big clubs in existence.
I hope they can find their best performance again, to be honest, I miss how extraordinary their performance was and could even be one of the feared clubs in the past, when they were in their glory days.