In last decade and even longer, Manchester United was completely a newbie on transfer market by purchasing players with super overvalued and expensive transfer costs. It is more painful as later they did not use those expensive players well enough and made it like doubled failures.
All clubs can make transfer mistakes that barely happen while with Manchester United, they have been doing this in many latest seasons. Hence, it is no longer mistakes but more likely severe systematic problem from their club strategies for development and necessary actions on transfer market.
The mistakes made by Manchester United have been going on for a long time, not just in the last 2 seasons but they often buy expensive players then their market value drops even though they only have 1 season with the team.
I don't think it's the players' fault, for example Anthony who was immediately able to find his best performance when he was loaned to Betis but with Manchester United he seemed unable to develop and that was inseparable from his very bad team which had an impact on all the players.
There are so many things that Manchester United need to change if they want to rise and it certainly takes a long time to start the process, not instantly.