He wasn't coming off the bench exclusively, it was pretty much a perfect string of alternate games where he and Rashford started. He started and performed well in games vs West Ham, Leicester, Basel, CSKA etc. None of these teams were attacking us with 6 men either.
Tbf, I dont even know why you're debating using the "he's worse against teams that defend in numbers" argument anyways. Who are these attackers that are better when up against more defenders than less? Who are these attackers who are worse when they have acres of space?
This isn't aimed directly at you but caf's bloody mental at times when it comes to some posters who will try and explain every single thing that happens on the pitch. Its never as simple as a player having an off game or the opponent doing well on the night. Nope, that'd be too fecking simplistic given we are on a football forum and thus need to present our expert analysis which explains every "Why" out there.
Martial is a young footballer. Like other young footballers around, he has good and bad moments. Like every other footballers around, he's better if given more space and less defenders to deal with. Unfortunately, this isnt rocket science and thus opposition managers know this as well and spend weeks on the training pitch to make life tough for Martial and other attackers.
Alternatively, I could be wrong and there exist these 21yr olds who perform at a high level every game without fail, who perform better the less space to play in and who perform playing against 10 defenders instead of 6 in which case you should immediately list these names out to the club.