I believe that Amorim is a coach with high taste of quality players and it does not mean that the player must be a superstar. The management is desperate to see a new Manchester United full of strength and zealous to win trophies. United performance has been poor for so long and it doesn't tell good about them because they have lost their respect.
You are right, it's not compulsory that Ruben Amorim must sign superstars or already made players before Manchester United's performance can stabilize. If Ruben Amorim can have the eye to sign players that are dedicated to playing for Manchester United, I think Manchester United can start winning their matches in the Premier League even though they don't win the Premier League title. Manchester United's problem can't be solved in one season; Ruben Amorim's target should be how to stabilize Manchester United's performance next season.
It is not mandatory for the manager of Manchester United to add a superstar player to stabilize their performance but we should see how weak their performance became. In that background it is imperative that they need some players who will be able to show signs of improvement for the team very quickly. As you mentioned, Manchester United's problems cannot be solved in one season, I'm stating the inconsistency here because if they can buy the best players, maybe they will be able to cover everything well in one season. The English Premier League and UEFA Europa League seasons have ended as a result most of Manchester United's players' contracts have already expired, so it's probably right that they are now looking for new players.