He's 20 years old and has time on his side - he's talented and his pace/willingness to run at defenders, regardless of success rate can be exciting (and equally infuriating) and it's something that we don't have much of, so he has many uses, even as a developing talent.
But I just don't see him as a regular future starter for a 'top' team (whether it's here or elsewhere) and am yet to be convinced he's the 'real deal' ...he was never really touted as a 'star prospect', he got thrown in due to injuries and scored a few goals, which catapulted him into the limelight, including earning many England caps.
His record is decent but weighted heavily towards his first 10 games or so and has been woeful in many games.
This is all perfectly normal for an inexperienced player, especially a 20 year old - I feel his glut of goals at the beginning as raised expectations to a level that he can't possibly achieve (at least at this stage of his career) and we should all revise them from 'world class wonderkid' to a talented player that needs time and even then, may not be good enough for us.
I mean, you have professional pundits saying he should leave because at the age of 20 he isn't starting every game for a historically successful team that has just finished 2nd in a top league...it's insane - let the kid develop and see. It would be great to see him improve into a great player but he's got a long way to go and I think everyone, including himself, needs to calm down a bit.
I also think Mourinho has to share some of the blame here too - his constant public criticism of players must be partly why they (the likes of Pogba, Shaw, Martial) try to do too much and go for personal glory, when a team mate is in a better position. He doesn't seem to reward good games and we've seen it many times where good performances result in being benched for weeks after - it's no wonder they think they have to score every time they get the ball.