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Many coaches failed in Manchester United after Sir. Alex Ferguson retirement years ago. They actually spent a lot of money for player transfers and for coach compensations because of sacking them. However, there are something wrong strategically in Manchester United as they spent money not for positions they need to improve most. They bought players are expensive costs that are like overpaid and their player transfer strategy is not consistently recent years. They self create chaos in their clubs and hope that a new coach can help them to fix all systematic problems.
This hope is unrealistic and this task is impossible for any coach. Need more example, another club that is similar to Manchester United is PSG. They once spent a lot of money bailed to gain the success they wanted.
There is nothing wrong about spending a lot of money on players, but is just a matter of spending such an amount of money on the right and productive players, another issue with Manchester United, I think is the aspect of lack of coordination of the individual players among the team members. Also when signing a player you would look at the already existing players and also look at whether the incoming player will match with the players in the team, else a coach will end up signing a wasteful player and this is why you see some expensive players not do well when they are signed into a team because they are not suitable to that particular team, Using Eden Hazard for instance.