A cardboard cut out will be an adequate replacement based on last season.
If you check the stats you can almost deduce that whenever Rashford did well, Manchester United did well. The correlation might work the opposite way ie when Manchester United does well then Rashford does well as well. However there's no denying that prime United allow Rashford to shine and it relies on its goals and assists as well. Last season for example he made 10 assists, he scored 30 goals in 56 matches. That's remarkable.
That's not all. Rashford's contribution are often minimized as well. Even in his worse seasons like the previous one he did manage to score 8 goals and create 5 assists. That's just 2 goals less then Garnacho (same amount of assists) and in terms of goals as much as Eriksen, Antony, Amad and Martial combined.
Now let's make it clear I want Rashford gone. That's down to four things
a- He's not worth 350k a week. A player like that needs to be consistent on a week in week out basis. Sure dips in form will occur but it can't last months and it doesn't excuse him from turning up on the pitch/training and actually work hard
b- He's a thin skinned player. whose affected by everything around him. That's lethal at a top club where the scrutiny is constant and criticism is unavoidable
c- Rashford seem only able to operate as a focal player (ie team built around him) but he can't be a focal player because of B. Focal player need to carry the squad including any criticism thrown at it. Think of Eric, Ibra, Messi, Ronaldo or Baggio. They are the team. Rashford can't do that.
b- I am not a big fan of the focal player in the first place. It makes the team predictable as everything will need to pass through him
Yet there's no denying that if Rashford leaves then he needs to be replaced. That doesn't necessarily mean like with like. The squad can be adapted with a better striker coming in, more solidity in midfield and defence, better wing backs and a lesser but still effective winger. However we can't allow Rashford to leave and not reinvest that money. This is not Manchester United 2010-2011 squad let alone the 1997-98 one