This piece covers a few bases.
https://www.football365.com/news/ole-gunnar-solskjaer-solution-man-utd-opinion
Perhaps the issue isn't solely recruitment, injuries, the Manager, the quality of players, but actually a little bit of all of them?
While i think there isn't a part of the club that is faultless, if you actually bought players to improve the squad, even more so when you sell players, then you will go a long way to improving things.
Obviously the players need to be ones that the manager wants and for a price and wage that fits the desired business structure, but somewhere along the lines, the short term business aspect is going to have to be somewhat sacrificed for the long term benefit of the club.