The thing is that it won't though. Rashford has world class potential as a Striker, but he's a mediocre LW. We are keeping a player with actual World Class potential as a LW on the bench to give Rashford playing time. Martial always looks dangerous with the ball. Leads the league in chances created per minute he's on the pitch. Rashford only excels when he gets behind defenses which would be useful in away games against bigger sides when you also need his better defensively workrate. When you need to break down smaller sides you use Martial's superior creativity and dribbling talents.
But Mourinho doesn't follow this logic. Rashford is the starter and Martial is the Squad player. Idiotic move in my opinion especially as players like Martial are far more rare than the Rashford's of this world. If you had to pick between the likes of Ronaldo, Messi, Hazard, Sanchez, and Neymar vs. the likes of Suarez, Benzema, Lewandowski, Lukaku, Ibra we all know the first group of players are far more special.
Martial should just leave. As much as I want him here, he's being wasted under the likes of Mourinho. Rather see a special player blossom than waste away. He needs consistent playing time to improve his decision making which he struggles with as a young player. I'm sure we'll spend 250 mil to buy him when he's a stud and Rashford is another decent, but overrated English striker.