I don't see what the big deal would be in him being in the NHL this season. Let him play wing/center on our 3rd line with 2nd pp duty, and send him to the WJC. Playing an NHL schedule will also help him cut down any excess weight. This guy is good at literally every part of hockey except maybe skating... his skating is slightly below average. He needs to learn how to adapt to the NHL pace and what better place to do that than the NHL itself. Another year in junior and it may elongate his problem with pace.
Kesler, Bergeron, J. Staal, ROR, Couturier, Monahan are all 2-way centers that made the NHL immediately after being drafted (30 games in Kesler's case). They have not "stagnated" or had their had their development go off the rails, so I don't see why Horvat can't come into the NHL and continue devoping now that he's already played his draft+1 season and showed considerable development.
I can think of one thing that may make me think twice about keeping him up in the NHL. Keeping him away from a potentially disastrous season this year.