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True. The decline of Man United that is happening now, it feels unfair if we only blame the Manager, because as you said individual mistakes are also the main factor that makes Man United like it is now.
To achieve success, basically a team needs effort, hard work and a large investment.
Manchester United may have tried to do as well as possible, it's just that some fans can't wait for that glory to come again. Manchester United have done this in the past, but there was a decline in performance after Sir Alex retired.
The huge pressure both from inside and outside on the possibilities really affects this club, that's why sometimes some of the best coaches are reluctant to coach this club. Not only does the performance of the team and its coach need to receive attention, but the club management should receive attention. What they have built so far for the club has in fact not produced the results as expected, there should be a good improvement in their efforts to improve it.