Just two months into his tenure, Amorim conceded this week that his job is on the line unless results begin to improve.
The Portuguese has won just two of his first eight league games to leave United languishing down in 14th, just seven points above the relegation zone.
"That is really clear, so we have to fight," said Amorim on United being dragged into a relegation battle.
"It is a really difficult moment, one of the more difficult moments in the history of Manchester United and we have to address it with honesty."
Alexander Isak and Joelinton struck inside the first 19 minutes as a fifth consecutive win lifted the Magpies into fifth.
Amorim stood aghast at what he was witnessing in the first half as Newcastle cut through the home side at will.
"They are a better team and they started very strong," added Amorim.
"When we suffer a goal it is really hard to turn around because of all the losses we had before and we don't have the base of work behind to cope with the difficult moments."
Just over three minutes were on the clock when Isak was afforded space inside the six-yard box to head in Lewis Hall's cross and score for the sixth consecutive Premier League game.
United badly missed the presence of suspended captain Bruno Fernandes as the Newcastle midfield three of Sandro Tonali, Bruno Guimaraes and Joelinton played around the ageing duo of Casemiro and Christian Eriksen with ease.
Isak blew a huge chance to double the visitors' lead when he fluffed an attempted dink over Andre Onana and the Swedish striker also had a goal ruled out for offside.
However, it was just a matter of time before Newcastle added a second and it arrived when Joelinton burst in to meet Anthony Gordon's cross.
Zirkzee hauled off to cheers
Amorim responded by replacing Joshua Zirkzee with Kobbie Mainoo after just 33 minutes - a move greeted with cheers by the frustrated Old Trafford crowd.
Tonali should have made it 3-0 when he hit the post at the end of a flowing Newcastle move through the heart of the United defence.
However, United did have chances to get back in the game before half-time as Rasmus Hojlund and Casemiro failed to hit the target with just Martin Dubravka to beat.
Amorim's half-time pep talk did at least muster some response as United began the second half far better.
Harry Maguire's header came back off the post before Hall blocked Matthijs De Ligt's goalbound follow-up effort.
Marcus Rashford had been restored to the United squad for the first time in five games but remained on the bench for the full 90 minutes as Amorim turned to another outcast in Alejandro Garnacho to make a difference.
But the Argentine international was introduced just as Newcastle had ridden the storm and began to assert their control once more.
A third consecutive home league defeat for the first time since 1979 is the latest in the list of sorry statistics that have marked United's decade of decline.
The fear for Amorim is that the losing run only looks set to continue into 2025.
A daunting trip to runaway league leaders Liverpool awaits on Sunday before United travel to Arsenal in the FA Cup third round.
