I think mental toughness will be needed if he want's to play for us again. He's definitely got the ability to be one our best players and regular starters, he just needs to find more consistency. With KDB and Lukaku both players didn't want to fight for their place as they believed they were good enough to start week in and out elsewhere, and they were proven right. I feel if they had stayed with Chelsea they would have also been successes. If Martial fights for his place he will start week in and out once he gets the levels he was at last season.