If he was any other team with any kind of depth he would be playing second line at this time in his career, in fact this would probably be his second year in the league.
He is going to be a fantastic player (way above average), you can't expect 21 year olds to dominate, especially when they were physically immature. Nuge is only 6 months older than Yakupov, he was very young for his draft year.
You keep stating that you can't expect 21 year olds to dominate(not that I was stating that I expected him to), like this is somehow not something you see from 1st overall draft picks.
Hall(PPG by 3rd season)
Tavares(30 goal/PPG by 3rd season)
Stamkos(50 goal/90+ point player by 2nd season)
Kane(Consistent 70+ point player)
Crosby
Ovechkin
Nash(40 goal scorer by second season)
Kovalchuk(immediate impact)
All were in the NHL at 18(excluding players with fake birthdates), all were on crappy teams with poor depth(typically the standard of depth a team needs to draft first overall), and all were at various levels of dominance early. There isn't a single forward drafted first overall in the decade leading up to Nuge's selection that would have qualified as anything less than a first line forward on every team in the NHL by their fourth seasons. Your statement that "He would be on the second line of any team that had depth at this stage of career" suggests that even you realize he's not as good as advertised.
Just asserting that you know he'll be good, and throwing out some token age and physical maturity excuses(which are both bull**** at this point), does not make for a compelling counter-argument in the face of RNH sailing in way below expectations for a first overall draft pick.