I don't think him not being called up will impact his decision to leave the club.
Martial playing more at United would obviously have helped his NT claim but he lost the spot off his latest friendly performance against Russia and the lack of commitment he's shown when wearing the NT shirt.
I still firmly believe he deserved to be in the squad because he's still having a better season than Lemar and is the more talented of the 2, still It's neither outrageous nor a big deal that he isn't in the end.
I wish he would stay but right now for both his personal development and United's future I think It's better if he goes, Jose and him it just cannot work, Jose is not going to leave the club anytime soon so if someone has to go It's Martial.