If you look at his shot map for Arsenal (bottom of the page), https://understat.com/player/498 you can see that the vast majority of his goals came from shots taken from a central position, typically inside the width of the posts, or marginally outside. He scored tons of goals inside the 6 yard box, and quite a few centrally just inside the area. While he took a lot of shots from wide left, very few resulted in goals. He also scored a few free-kicks from just outside the area, typically inside the D. For Arsenal he averaged between 3.59 and 4.12 shots per 90 mins over his last 3.5 PL seasons. At United 1.62 last season and 2.28 this season. Key passes per 90 at Arsenal were between 2.17 and 2.49 apart from his last 6 months, when they were 3.04. At United 2.22 last season and 2.73 so far this season. His expected assists per 90 mins this season is surprisingly high at 0.47. Last season for United it was 0.21. At Arsenal it fell between 0.25 and 0.32 during his last 3.5 PL seasons. In the FA Cup games this season he scored from his only shot against Arsenal and had one key pass. Against Reading one shot and one assist, no other key passes. If you compare with Martial. Under Solskjaer Martial averages marginally less than 2 shots per 90 mins. For the whole PL season he is at 1.99 shots per 90 mins. Last season 2.78, the season before 2.43. So in effect, we have bought a player whose physical peak was probably when he was 26-28, then asked him to perform a role that looks similar on paper to his Arsenal role but in reality makes it far more difficult for him to score the type of goals he was used to scoring.