#1 Impediment in Rashford's development
The fact that United have the likes of Alexis Sanchez, Jesse Lingard, Juan Mata sees Anthony Martial and Marcus Rashford battle it out for game time.
Jose generally started one of the two before acquiring Sanchez from Arsenal in January. The Chilean's arrival saw Anthony Martial move to the bench. Also, Marcus Rashford, who used to come on as a substitute for the Frenchman, did not feature a lot since then.
In a normal scenario, the manager has to ensure a fair set of opportunities to both. Clearly, Martial is not happy with the role he has at the club, but Rashford is younger and willing to fight for more.
Now that Martial is willing to leave, United should not stand in his way. After his departure, Rashford's game time would increase and he can be expected to improve.