That's the thing. This idea that he played enough and did nothing with his time seems to be prevailing, but after Sanchez was signed last year, he only started 5 matches the entire rest of the season. Only once did he start in back to back matches and those were two weeks apart on either side of the February international break. You just cannot expect consistency out of a player who never knows where he stands from week to week. He'd scored three matches running when Sanchez joined and despite the entire world seeing that nothing was coming off for Sanchez, he started all but two games the rest of the season no matter how badly he played. To be unceremoniously dropped for a player that isn't playing nearly as well as you were will weigh on a player -- especially a young player.
I can accept that he's been completely unprofessional in the way he's gone about showing how unhappy he is, but I still think he was horribly, horribly mismanaged by Mourinho. We'll no doubt sell him on when we can get a replacement in, but it's sad seeing how much potential he had and seeing what the club has done with it. Blaming him alone is easier an easier pill to swallow than holding Mourinho accountable for running off yet another exceptional young talent that he didn't think was worth the effort to develop (Salah, De Bruyne, Lukaku, Bonnuci, et al).