I agree to the extent he can say what he likes. However I think players who are clearly in our future plans sounding off here and there, which could annoy fans and could lead to disciplinary action from the club, is one thing. Players like Matic who haven't consistently performed for us in over a year making disgruntled noises about not being picked is another; fans cannot take seriously players who do all of their talking off the pitch and are naturally inclined to give more leeway to players who can potentially do good things for us on the field.
Matic had no reservations because he knew that others before him had done the same and they did not receive any kind of disciplinary punishment. This is the core of the issue here. The club chose not to make it clear that the only authority which can pass judgement on the manager is the board. Pogba liked posts which were welcoming his move to Madrid, Shaw liked posts made from his siblings that were slating Mourinho, Martial did exactly the same thing as Matic and nothing happened. They are all first-team regulars, this was their reward.
As for the fans, i think that they should not be OK with players crucifying the manager openly no matter what or whoever the manager is. Especially when said players have played their part into turning Manchester United into a midtable club. All of them. Otherwise, we should all agree that Lukaku was 100% right when he stated that there are favourites in our dressing room who can get away with murder while others will always be hanged out to dry.
But for some strange reason, our fanbase felt extremely hurt when Lukaku made such assertions. It seems he might have had a point, after all.
