Not that Manchester United don't need long term consideration with coach, but it depends the agreement the coach entered with the team management that will make the management to give the opportunity to the coach, for team to see what he will help the team to achieve at the end of the season, once the management know that the coach will not make it with his promise with the management, then sack letter will follow to remove the coach without considering the short time the coach has spend in the team, i think Manchester United coach will help the team to perform well in this season because they displayed perfectly well in their first matc that make people to believe that Manchester United will reach top five in premier League table.
Manchester United has always fired previous managers if management deems them incapable of delivering their best.
Some previous managers have even managed to deliver trophies, but in reality they were also fired for reasons of inability to deliver good results in the Premier League.
Before achieving success, Liverpool under Jurgen Klopp also struggled, but management didn't fire him and instead supported him, ultimately leading to his success for the club.
Building a team to achieve success in a short time is no easy feat.
It requires time and opportunity for the coach to develop the team to its full potential, and it requires encouragement and support from various parties.
This is what Manchester United management did not do after the Ferguson era ended.