Well, his predecessor, Erik Ten Hag just got sacked from Bayer Leverkusen only after 2 games, so I don't know why Man United is waiting that long  
Leverkusen's sporting director said that they felt things were going in the wrong direction, so they decided to act quickly rather than wait till the situation gets much worse. Maybe United should take the same approach. If there's no chemistry between Amorim and players and if there are no clear signs of improvement, maybe it's better to call it a day and let him go. But they know more than we do, so if they think Amorim has a plan and they're satisfied with it and they are willing to wait for the results, they should give him more time.

I'm not sure what's the sentiment towards him among the Man United's fans, I think there are mixed opinions, some want to see him replaced, while others are happy to give him more time.