I doubt he comes in for the Dallas game, probably not St. Louis either. Both those teams are larger so it's possible they don't want him out there when he hasn't readjusted to the speed of the NHL game when hits are coming. I feel like the team has stepped up its physicality since beginning to play better, though the degree of that isn't of great significance. Still, likely to see a lineup that can handle that better and I'm not sure you want Hayton at risk.
I also really doubt his development is being drastically affected by sitting another game or two. I mean, honestly, there's no expectation from any side that he'll carry the team into the play-offs once he's in the line-up. He's going to be focused on improving his game and adjusting to being a full-time NHLer so he can be on the full-roster come next season. If he excels, great, but to act like this is mismanagement of the highest order is hyperbole.