It wouldn't be wrong to say that Sporting had a fantastic season with Izban and won the championship thanks to him. However, the Premier League is a completely different level. Many managers in the past have won their own leagues but failed to compete in the Premier League despite being very good. Ten Hag, Amorim's predecessor, is one such example. Perhaps Manchester United truly needs an experienced manager, as they say. It's not wise to take risks with a manager.
I have been talking about this for a long time, Manchester United have chosen a very strange tactic regarding coaches, they are trying to hire coaches without great achievements in the top leagues and for some reason they think that they will be able to achieve something, especially since the team is clearly in crisis now. They need a top coach, and maybe they avoid it because they know that such a coach will demand huge amounts of money from them for new transfers, and they are not ready for this now.