In Jose's second season he was originally behind Rashford in the pecking order on the left but he out performed him and won his place. We then stupidly signed Sanchez and replaced him.
The following season he reclaimed his place from Sanchez. He was one of the top performers during a period which kept Jose in the job for longer than he deserved.
Ole was then happy to sell Lukaku to use Martial as striker.
Last season you can argue that he downed tools due to competition with Ronaldo, but I think that's understandable when Ronaldo would always start regardless of form.
If you dislike Martial, fair enough. If you think he isn't good enough for us as starting striker, then I agree with you. However, this narrative of him being unable to accept competition is evidently false.