It's purely about form with Martial for Deschamps. There's a lot of armchair analysis of Deschamps in regards to Tony, but he has nothing against the kid. At the Euro, he was simply exhausted and out of form. He was not ready for that tournament, which motivated Martial to work on his endurance more as he's lacking there. When he was stringing good performances before Sanchez came, he was getting reintegrated and I'll argue he could have very possibly made the squad had Sanchez not come and taken his spot without deserving it. The injury wasn't long enough to undo the good work he had been doing. But once he was relegated back to the bench and playing sporadically, that was going to be the nail in the coffin for his chance of making the world cup.
It's just the way it is, I don't think the vitriol against Martial possibly leaving is warranted because he did come back from a long way out last season after being the team's highest scorer in his first year. There's just something not right with Mourinho and Martial and it's got to end because the guy is clearly not being treated the same as the other players by the coach despite being one of the team's most productive player. Now, we could all be in for a shock and Martial ends up staying and they find a way to work it out, but I really doubt it and maybe it'll be for the best for both parties to call it a wrap this summer.