But his struggles actually extend all the way back to Euro16 where he struggled to get a game.
They are many other reasons why he could be struggling :
He's had family problems, possibly affected by the shirt issue earlier in the season which might have left him feeling undervalued, possibly affected by a change in management with a manager that has shown less confidence in him than the previous one.
There are so many other reasons other than simply sayings it's because he is struggling with the physical demands of the league. He didn't struggle with this physicality last season and Ligue 1 is one of the most physical leagues in Europe.
It's such ill informed opinions which always seek to find ways to glorify leagues instead of realising that they are plenty of other factors at play which are problematic.
Ill informed opinions? You've taken a good season from Martial last year to conclude that he's not having a problem adapting to the league, ignoring he's been poor for us throughout this season. That's wrong whichever way you look at it. Alfonso Alves was having a belter of a debut season for Middlesborough and he went on to be rubbish in the next season and throughout his entire career.
I agree that there are other contributing factors, such as confidence or system or mentality, but you cannot ignore players pay a special attention to him this season and his work is cut out.
Back to the point though - Zlatan has done very well but he has not single handedly destroyed the notion that players from outside leagues struggle to adapt here (the same can be the case for moving from PL to other leagues).
People such as yourselves appear to believe a struggle to adapt means failing to make any form of impact during their time in England. That's not the case. Its simply a case of them not being able to hit full gear as easily as they did in leagues where they were more comfortable in.
Examples of ex united players players alone initially struggling to adapt in England include: Veron, Di Maria, Depay, Forlan, Pogba, Evra, Vidic, DDG. Now some of these players went on to become legends/settle into their potential from 2nd season onwards. Others just flopped full stop. But all of them struggled in their first season. Zlatan scoring in his first season doesn't take that away.
How did the physical demands of the premier league change in one season for Martial?
Players are paying more attention to him this year than last.