Alexander Isak and Joelinton scored early goals for the Magpies, putting the home team in disarray. Amorim made a controversial decision to substitute Joshua Zirkzee in the first half, receiving negative reactions from his own supporters.
It could have been more for Newcastle, Isak having a goal ruled out for offside and Sandro Tonali striking a post. There was something of a response in the second half with Harry Maguire heading against the post, but it was a deserved win for Eddie Howe's team.
Manchester United have now lost five of their last six Premier League games and four in a row in all competitions, the last three coming without even scoring. They remain 14th in the table, now only seven points above the drop. Newcastle climb to fifth.
"With bad results, it's really difficult to sell an idea," Amorim told before the game. "I was hired because of that idea and in these difficult moments if you change your mind, it is the end for any coach." But sticking with that idea brings problems too.
TrendingManchester United were a mess from the outset against Newcastle. Some of it is down to players bereft of confidence. The sad sight of Zirkzee being booed from the field before the break was emblematic of a group of players who look unhappy in their work.
But a system is supposed to make their jobs easier not harder and whatever your view of the quality of the squad inherited by Amorim it is increasingly difficult to argue that this is not at least part of the problem. Too many of them look uncomfortable in their roles.
Diogo Dalot was stationed so high up the pitch for much of the game that it would have been more natural to have a left winger there rather than an erstwhile right-back. When he did get the ball he could only shovel it inside rather than provide any genuine width.
Noussair Mazraoui fared little better on the other flank and Matthijs de Ligt does not want to be dragged into wide areas as he was by Alexander Isak. Lisandro Martinez is one of the few this shape is said to suit and he endured a wretched evening in red.
Amorim would no doubt have preferred to have come into this job last summer or next rather than be plunged into a congested fixture list and asked to fix problems on the fly. But the hope would have been that he could muddle through and build for next season.
That remains the hope but it surely needs United to pick up or risk beginning that rebuild on the back foot, being doubted even before his first full season in charge. As he himself acknowledges, there is an idea to sell - to the players, to the public and to his bosses.
Amorim does not need a trophy. A season out of Europe could even make things simpler. There is no expectation United fire. Just function. But a third Premier League defeat in a row without so much as scoring a goal is unacceptable even at this low ebb.