A couple points. Willy and Marner spent a lot of time on the 4th line as rookies and it didn't hurt them. Also, if Robertson can provide what Johnsson/Kappy can for 1/3 of the price its a no brainer.
Here is the list of what players did at 19 after having a similar 18 y/o season to Robertson:
1. Kane - Age 18 - 2.50 ppg - Age 19 - 0.88 ppg in NHL
2. Marner - Age 18 - 2.04 ppg - Age 19 - 0.79 ppg in NHL
3. Dylan Strome - Age 18 - 1.98 ppg, Age 19 - 2.14 ppg in OHL
4. Raddish - Age 18 - 1.88 pgg - Age 19 - 1.43 ppg in OHL
5. Matt Tkachuk - Age 18 - 1.88 ppg - Age 19 - 0.64 ppg in NHL
6. Nicholas Robertson - Age 18 - 1.87 ppg - Age 19 - ???
7. Taylor Hall - Age 18 - 1.86 ppg - Age 19 - 0.67 ppg in NHL
8. Tavares - Age 18 - 1.86 ppg - Age 19 - 0.66 ppg in NHL
9. Galchenyuk - Age 18 - 1.85 ppg - Age 19 - 0.48 ppg in NHL
10. Viladri - Age 18 - 1.81 ppg - Age 19 - injured
And here is the list of 50 goal scorers.
1. Robertson - Age 18 - 1.20 gpg - Age 19 - ???
2. Kane - Age 18 - 1.17 gpg - Age 19 - 0.88 ppg in NHL
3. Tavares - Age 18 - 1.04 gpg - Age 19 - 0.66 ppg in NHL
4. Debrincat - Age 18 - 0.85 gpg - Age 19 - 2.02 ppg in OHL
5. Toffoli - Age 18 - 0.84 gpg - Age 19 - 1.53 ppg in OHL
6. Fabbri - Age 18 - 0.83 gpg - Age 19 - 0.51 ppg in NHL
Ultimately it is up to Robertson to earn it in camp. We have no need to force him into the lineup if he isn't ready. But there are a ton of precedents for kids doing what he is doing at 18 and jumping into the NHL as legit top 6 forwards at 19. He should get an opportunity to prove if he is ready in camp. And probably even the 9 games.
He has put up a legit elite seasons. The only asterisk is scoring may be up in the OHL this year as Rossi, McMichal, Byfeild, Perfetti and Kaliev have also put up legit elite seasons this year. Although, even if you discount his goal scoring by 25% its still the 3rd best goal scoring season by an 18 year old in the last 20 years behind only Kane and JT.