Amorim was brought in for longterm and he will have 2-3 seasons at least. He got the position in the worst possible time and with this kind of players he is doing the maximum. If he is able to win the europa league this season will be a good season for them. This is the only thing that can save their season so far. Even if he dosent win it i dont think he will be sacked.
The Europa League this year for United is something that can turn the season around and turn it from unsuccessful to successful, since getting the title + a ticket to the Champions League while being in the bottom half of the standings is the maximum possible. Thanks to the new format and low competition, this is quite possible - United's chances are currently estimated at around 18%.
That is true, if they could somehow manage to win the Europa league title then Amorim will have a lot more time and he will be considered great. However, Mourinho won this and yet he still was fired because more than anything else, the league title matters, and how it is going at the moment matters. Look at ETH, he was a terrible manager, but he has a third place, an EFL cup and a fa cup under his belt, even though he won all of those, he is still considered a terrible manager.
Because the time he got sacked, United wasn't even in top 10. Which means, even if you win something, in this case Amorim with Europa league, if next season they are still not even in top ten, he will be fired too. He has to do better next season.