Some can argue that Martial is now starting to play the way Mourinho wants so that's why he's getting this run of game and playing well. We can see an improvement on his work rate defensively, perhaps taking training more seriously, and that he couldn't perform before because his stamina wasn't up to par to be both effective and dangerous offensively without being anonymous defensively. I would argue that it was far more foolish to choose to sacrifice months of development and what he brought offensively on this team because the defensive work rate wasn't up to snuff, especially when he had a big impact on goals and results prior to Sanchez coming in for his spot. The other perceived improvements on his game, like his running, motor, and decision-making is not something I agree with. I think he showed that his very first season here. However, I agree he played uninterested at times as well. He is playing with more confidence, and with three years now at United, he is more effective than he was at 19 years old, but the overall game is the same.
The reason why Martial is succeeding now is the same as why he was succeeding in last season, he is feeling confident and he is in a rhythm, something that's been difficult to get under Mourinho. He also now has the benefit of an in-form Luke Shaw behind him, and even though Rashford wasn't great, he is also offering more movement and opening up more spaces for Martial to attack than Lukaku. So the two latter things are something that benefits him even more than last season. But when I hear Mourinho say Martial is now doing things he couldn't do before, all he can be talking about is the defensive work-rate, nothing about the goals he's scored or the impact he's had offensively is new.