An unfortunate disappointment so far. I do think not having a run of games has hurt him, at the age he was at when we bought him he needed to keep playing in order to develop. He had a run of games early on under Van Gaal and did alright then he didn't play that often but had the very occasional good performance when he did.
Ultimately he doesn't or hasn't fit into the styles of play that our managers have had. He's at his best in a team which play quickly on to the counter where he can run with the ball into space and also run in behind. Under Van Gaal this was rare as we allowed teams to regroup and get numbers behind the ball, Memphis isn't the type of player to go past 3 players in tight spaces, he needs to be running onto the ball in transition. If teams have men behind the ball he's fairly limited as the only thing he can really do is cut inside and shoot/cross.
You'd have thought he would be more suited to Mourinho's system but I think its clear the Mourinho of now is not the Mourinho of the past, at least the Mourinho in his first Chelsea spell up until Madrid. Remember the times when Mourinho would do things like leaving two players forward when defending corners? I don't think he's done it in a long while. He's become even more pragmatic than he previously was as he expects his winger to get back and defend every time we lose the ball and again this just means we don't have much scope for counter as all of our players are forced back. It's clear this was the issue in the City game where we had nothing in behind from the wide areas or centrally and ended up just having to pass the ball about 5 yards which just played into City's hands. This is something that Memphis wasn't forced to do at PSV, he was allowed to express himself and perhaps stay forward at times. I think this also is the reason people like Rashford and Martial have struggled under Mourinho.
He's been brought on late on in games and not been great. I think we as fans expect a young player coming on late to basically run around and give absolutely everything he has and be almost Tevez like. I would agree with those who say that he seems like he doesn't really care when he comes on, that's clearly on him but there must be a reason for that. Perhaps it's just an attitude thing, perhaps he feels like he's not wanted. It's impossible to say really, I just hope that rather than just giving up on him Mourinho will put his arm round him and tell him that even though he hasn't been good enough so far, he has faith that his quality will eventually come through and will start him in a game or two and allow him to express himself, whether that's in the League Cup or the Europa.