I have been one of Mourinho's biggest critics recently, but I'm actually willing to cut him some slack in this particular case. Martial is not entitled to anything - if he wants to be a permanent starter, then he has to start improving his work rate and overall desire.
Yes, Sanchez has been underwhelming, not denying that, but the point remains, he still at the very least puts up an effort. And let's not act like Alexis hasn't had some great shifts (remember his hardworking performance at the Etihad?).