One fact is that Manchester United have good-performing players that can still play for them next season even without buying more players, but the main thing that Manchester need now is a better coach that will be able to manage them and put each player in their best position. Manchester United will have to bring a better strategy that will help them sustain their reputation even though they will be able to make it to the top 5 in the Premier League next season, as we all know that they will not be in any other league again unless it's the Premier.
As all clubs, there are good and bad players in their club so it's wrong to say Manchester United don't need to sell or buy any player in a coming transfer season. You can give me only one example of a club that has no player in and out in a summer transfer in history. I bet that you can not find any example like this but let's say frankly that if you can find one case, it does not enough to convince me that a club with a lot of money and in performance crisis like Manchester United won't need big changes in their player resource this summer.
They actually need to buy not only one but several players and there are also players must go out of their club in this summer. Ruben Amorim won't stay to work with current players and if Manchester United told him such a boring transfer plan, he would leave this club already.