I'd be less concerned about +/- for a young offensive talent, and more concerned about his ability to create chances, get to scoring areas, and play engaged hockey.
The latter is what has most viewers concerned. Teams have adjusted to his game, and he really hasn't adapted at all.
His 5v5 iHCDF/60 has been cut in half, from 3.05 to 1.53. That's a staggering fall off, especially considering the team around him has improved.
Some of the raw all situations numbers are really ugly as well.
Last season (68gp) vs this season (81gp)
He's creating a lot less chances for himself
206 vs 191 shots
69 vs 44 iHDCF
24.0 vs 19.3 ixG
He's drawing less penalties and taking more
25 vs 13 penalties drawn
14 vs 24 minor penalties taken
+11 vs -11 minor penalty differential
He's giving the puck away much more and taking it away much less
57 vs 90 giveaways
47 vs 25 takeaways
-10 vs -65 differential
And he's been much less physical:
54 vs 47 hits given
77 vs 58 hits taken
All these numbers point to a player who has been effectively kept to the perimeter and forced to settle for low probability passes/dekes. He just hasn't been able to penetrate the middle of the ice on a consistent basis, and seems more hesitant to engage physically.
He needs a big offseason in terms of physical development, but also needs to do some searching to figure out what he needs to do to successfully adapt his game.