I don't think it would be difficult. Very few teams can afford his wages, Barca and Madrid are stacked for players in his position, as are Man City. Arsenal would only let him go to Chelsea if they got Hazard in return which won't happen. Unless he genuinely wants to stay with Arsenal (which all indications suggest he doesn't), the stars could aline.
Personally I think Martial should be kept, as his potential is huge. But history as shown that if a Mourinho doesn't like the cut of a player's jib, no matter how talented, he's prepared to move him on.