Not sure why the 3421 is talked about as this uniquely Amorim thing. Doesn't most top teams on the planet play this shape? Certainly at least flirted with it in the last couple of years. Pep certainly has played a 3-4-2-1 since the treble season, and Leverkusen under Alonso basically played the excact same shape last year. I am sure there is differing tactical instructions which better tactical minds than me can explain, but from what I understand it is all about that midfield box of four, while still having width and a forward to stretch defences, while maintaining numerical superiority in the backline to always have a free man for bringing it out of defence. Basically a shape and ideas most team and managers now implement into their gameplan even if they stray a bit from the formation.
So I am not sure why the 3-4-2-1 is so controversial at United when Amorim uses it. It is basically currently the default formation just like 4-2-3-1, 4-3-3 and 4-4-2 has been in the past. He just needs players that understand what they are supposed to do, and have the confidence and ability to execute it. As you need in any formation, but at least the one he is implementing matches what most other teams are setting up like