The manager is not able to bring about very good performance with all the players that are in the current team of Manchester United. The team may have big stars but these stars are not able to do much for the needs of the team, as a result of which it is seen that Manchester United is regularly performing poorly in the English Premier League and other competitions. Of course, changing some players in the team has become a very important issue.
You can see PSG with and without Mbappe as well as their differences in operation. With Mbappe, they did not have proactively defense at their front line and in matches when Mbappe could not play well, did not score goal, his contributions for PSG was nearly zero. Without Mbappe, other attacking players in PSG are ready to do defense proactively and even they don't play well in attacking, they can contribute a lot of things for a team defense. Dembele is an example for how a main attacking player can contribute for their team defense and together they win Champions League title.
If Manchester United starts the new season with the same team like every time, then it will be seen that they are again ending the Premier League and other seasons with failure. If they do not want to change much, then they will still have to change some of their defenders, midfielders and some strikers. Even if they change four to five players, Manchester United's performance will return to good condition.
Manchester United have many good players but they don't have balance between attack and defense. They must have better defenders, more and better defense from their front line to midfield so that together as a team, they will be able to cover their goalkeeper more. It will help them have more solid defense that is a foundation for their success.