Some players just aren't cut out to be two-way guys and I don't think there's anything wrong with that, nor do I think it has anything to do with compete level, like some people like to insinuate.
Sure, there are guys out there who just don't want to backcheck but there are also guys out there who lose themselves in the drive to score and, for whatever reason, aren't able to flip the switch from offense to defense as fast as necessary. I think for a lot of those guys, demanding two-way play is either a near fruitless endeavor or reduces their effectiveness offensively, thus reducing their effectiveness overall. I don't think guys like Tatar or Holmstrom or Hudler are/were never going to be stalwarts defensively because they didn't try to be but rather because they just didn't have the natural instinct for it.
Now, I'm not saying Mantha should be given a free pass but I sure as hell don't think we should wait around until he develops some notable two-way game down in the AHL. He's going to have a specific role at the NHL level and those kinds of players don't need to be a jack of all trades if they can dominate one.
Also, he's very Franzen-esque in his body language, so its hard to get a read on his compete level.