Marner is a complimentary player, not a star. He was great in Junior because he has skill, but Junior is a 'Soft' league and Marner could be a star in a soft league. He reminds me of Joe Juneau in his style of play - a passer instead of a scorer. And with Marner, 40% of his points last year came on the Power Play. So, at even strength, he isn't overly productive and he is never going to be a big goal scorer, as his shooting percentage is 11%. He is decent on the face-off though, as he won 52% of his faceoffs taken and he isnt really a Centre anyways. That is quite impressive actually.
The NHL is a very tough league. Not as tough as it was in the 1990s or the 1970s too, but still, bigger guys that hit harder and push players around much more than in Junior are everywhere in the NHL. Marner only scored 14 goals last year when the game was at even strength. 14 goals when skating 5-on-5 over an 82 game schedule is not impressive at all. His plus minus, at - 1, was also not good considering he got lots of power play time.
So, Marner needs to play with a talented player in order to produce in a bigger, tougher league. He could never carry a line on his own. Marner is worth $6 million max. His 20 goals a season aren't that hard to come buy. Even Tie Domi scored 15 goals in a season when he played with Sundin and I don't think anyone can argue that Domi is a skilled, talented player. 15 to 20 goals can be had by anyone if they play with good linemates. I love Domi and he was a great fighter, but again, 15 to 20 goals isn't anything to brag about.