According to Louis Saha on the All Out Football podcast 'In The Mixer', Moyes' failure as Manchester United manager was partly because he took the job too early and failed to adapt to the demands of a squad full of star players.

When Moyes first managed Everton, he guided the team to fourth place in the Premier League in 2005, finished in the league's top eight nine times, reached the FA Cup final, and recorded 218 victories. In 2013, he replaced the retiring Ferguson as Manchester United manager.
Moyes was dismissed by Manchester United after only 10 months in charge. Although he won the Community Shield, United finished seventh in the Premier League due to poor results and missed out on the Champions League for the first time since 1995. After that, Moyes led West Ham to win the UEFA Europa Conference League and is now managing Everton again.
Saha said Manchester United's situation had already been affected after Ferguson's departure. Moyes needed to manage Giggs and other high-level foreign players, while he may not have had as many players of the same level during his time at Everton, making the transition difficult for him.
Saha believes Moyes needed to be more open and give players more freedom, but he was unable to do that immediately. Saha also said that, like Ferguson, Moyes loves football, is passionate, and wants to help every player, but Moyes placed more emphasis on systems and whether players fit the requirements, while Ferguson trusted players' talent more.
Saha played under Ferguson at Manchester United for four and a half years, winning two league titles, making 124 appearances and scoring 42 goals, before later joining Everton.