Thats assuming of course that rasbford has his hopes tied to the number 9 position. He played as a winger/10 at youth level.
And I'm not sure he is a 9 in the long term. I agree that he has been at his best for now as a striker. However, he has some flashes of Bale/Ronaldo, and we might underestimate how "underdeveloped" physically he is compared to these two at the same age. I was quite surprised when Mourinho claimed that he has grown an inch the past year.
I would assume that imply that they are still being carefull with him lifting weights, explaining a quite slender lower body. I can imagine his acceleration and body strenth will not be fully developed until 2-4 years, while several players already are at the age of 20-21. Gareth Bale sudden boost can, to a huge extent I believe, be explained by the strides he made physically. Considering Marcus Rashfords pace and size I can see him having a similar development.
He will probably also look less gangly and more comfortable on the ball when he stops growing. The way he strikes the ball is already impressive.
Of course, developing physically will make him more adept as a striker too. But I feel he might have even bigger value as a wide forward. The striker has to stretch the defence, work with his back towards the goal and do a lot of dirty tactical work even when his team are in possession. They are usually the player with fewest touches on the ball of any outfield player on the pitch.
Jose Mourinho probably does not care if Lukaku looks like a donkey at times; he will score goals and he will create space for his team mates. Unlike Zlatan he will not drop deep to participate in the build up. He will push high and create space for his team mate. Even though Zlatan is the better footballer I would not be surprised if Mourinho prefers a striker like Lukaku.
In the long term, I believe Rashford might be able to do more than creating space. I want to see him with his face towards the goal, on the ball running at defenders or attacking space ferociously.