Ruben Amorim has the potential to lead Manchester united to success this statement of yours is really funny because it now looks as if the other's managers that left has got no potential at all. I know it is always common to attribute blame to managers when his team are not doing or playing well, despite the fact that the manager is putting in all his efforts but since it is not reflecting on the performance of the team, the manager will be seen as someone that is not good for the job. Before Ronald left Manchester united he pointed out so many and he even complained of the facilities. So what are trying to say here, most of this challenges is not actually from the managers but rather from the management until there is a change in their policies Manchester united will continue this way even if they should change Ruben Amorim
All of the new managers/coaches have the potential to lead Man United be successful when it was in the early of the days they managed Man United. But after few matches, people will say the new managers to have no future in Man United. It includes Ruben Amorim, I've read many comments to change him with a new one. So, it is quite surprising if there is a person who stated Ruben Amorim to lead the success for Man United. 
Well, it is a very difficult job to manage Man United because the board and the fans have too high expectation on the new manager/coach. Moreover, the quality of Man United squad is rather worrying, many of them don't deserve to play at Man United. Before thinking of a high expectation on Man United performance, I think the board must consider to rebuild the squad with better quality players. But this will require a lot of money for signing new players, also Man United needs to brave to sell the current players with lower prices. I think Amorim will do this in the next season if he still stays at Man United.