I think he actually shares a lot similarities with Park, at least towards the latter years of Park's time at United.
Which is that he is played in a position that *should* be influential in creating and scoring goals, however neither really do it. Park had over 25 appearances for us in 4 consecutive seasons, yet only once contributed more than 5 goals (8 in 11/12). Never delivered more than 10 goals + Assists in a season. That was playing very much as a forward, often one of just 3. In 2 of those seasons, Nani had more than 20 goals + assists, and he was generally viewed as disappointing.
It's the belief that if you're playing as a forward winger for United, you should be contributing directly to goals. 5 odd goals+assists in a season is, frankly, terrible for that position. Even ancient, barely playing Wayne Rooney is contributing higher numbers than that.
So people can talk about the 'great positions he gets in' or his 'intelligent use of space' or his 'good combination play' til the cows come home. Football is about scoring goals. And he has shown nothing in his career to date to suggest that he's going to get us numbers.
If we want to get back to the very top, we cannot afford to have attacking players that don't score goals.