Its surely quite possible to think that Martial is being mismanaged without supporting him over the club? You can think, if we used him more / played him and Rashford together / played him up front it would make the team better, but Mourinho isnt doing it - and then complain about that fact. You can think it would be better for him as a player if he went somewhere else and played under a manager that would use him better and get the most out of him, surely that would be an objective opinion, not a case of supporting the player over the club? As Rozay said, you might have that opinion, it might frustrate the hell out of you, in due course maybe the player leaves - Martial, Kagawa before him, whoever - and then you move on.
Of course given the size of the fanbase there will be exceptions. Im sure there are people who will become PSG fans if we sell Martial to PSG - or whoever. But my guess is most people who say we should play Martial more, or differently, or Martial is the best player we have, are saying it because in their assessment it is in the best interests of the club.