I don't think the style of formation Amorim uses is the problem. His 3-4-3 formation works for him. We saw him use the same formation at Sporting CP and he flourished with it, even in the Champions League too where he used such similar formation to win Manchester City. I think it's the players that needs to adjust to the coach tactical strategy and formation. Manchester players are so weak and lazy, most players don't have the stamina to play in such a rigorous formation. For Amorim to effectively implement this formation, he would have to offload the current team and buy new players.
So far many persons have not been able to fault his style or tactical approach but the issue I think has always been around proper execution, sometimes, they try to execute it and that is why we could observe that there's a little change in their patterns but to completely execute to productivity has been one of the biggest challenges of the tema and with time they will hopefully get pass it. When he played against city with sporting, remember he's been with those players over the years and they get to understand his patterns so well it wasn't a challenge as they are getting with united. So basically he needs new players but not to completely overhaul the team.