Confused 19 or 20 years old?
Plus why is he rated so lowly?
Perhaps because of expectations rather than performances. There seem to be a number of players like that on that list. Thauvin for example is another one. There aren't many 19-20 years old players that score double digit goals in top 5 leagues, so it is unreasonable to expect that. Most strikers have their breakout season's aged 21-24, so its clear we have bought him in the hope that he has one of those in next few years.
Comparing his goal production to some of best young strikers at the same age we can see this.
Vietto- 5 goals in Argentine league
Dybala- 5 goals in Serie B
Harry Kane- 2 goals in Championship
Icardi-10 goals in Serie A
Lacazzete- 4 goals in Ligue 1
Nabil Fekir- 1 goal in Ligue 1
Jordan Ibe - 0 goals
Raheem Sterling - 7 goals in PL and he supposedly worth 50m pounds.