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.
You are right, growth is not easy, it doesn't happen over night, it happens gradually and goes through alot of processes, so the management of every football club must always be patient with their club managers, for even when a new manager is hired, he or she can not automatically already know each of the players capacity and abilities, it will take some time for him to learn this properly and know which and which players to push forward when ever the going gets tough, hiring a manager today and sacking him one year after due to underperformance doesn't make any sense.
But on the other hand though, every football club has their own rules and guidelines and I believe that some times, coaches don't just get sacked because the management of the club thinks the coach is underperforming, but may get sacked because the coach isn't following the club's rules and guidelines, even though such may not be made public.