Yeah, losing Zlatan must have ****** up Mourinho's longterm plans horribly. He was probably hoping that after another year Rashford can become the main man, at which point you just bring a moderately priced backup for him.
Now though? You basically need to splash huge fee for a new primary striker, but a guy like that isn't going to be willing to be a backup, while Rashford obviously isn't going to be a one-season wonder and has what it takes to become one of the very best, so you need to give him chances and develop him.