What separated Mcilrath from going from 7D to 2D on the first pair was the inability of the team to rely on him in one-on-one situations against speedsters. How good he becomes at that will determine if he'll spend his career making $600/yr as a garbage journeyman or $6/yr as a fan favorite.
That said, he was 23 the whole last season, so there's still time to get better, he just needs to focus hard on it. It's more than a month before the first preseason game, over 1.5 months before the regular season. He needs to find a speedy forward and practice one-on-one for hours, preferably under the gaze of a coach or a former NHL defenseman, but if not, at least do that alone. Eventually it becomes muscle memory when facing speedy forwards.
Someone should also point out to Dylan that a year from now he'll be 25 with 2 years in the NHL, so it will be more or less a WYSIWYG situation, and even if he somehow improves after that, it will be very hard to change the coaches thinking who'll assign his good play to being hot, rather than permanently better.
This is the last season when he can develop a lot and people still view him as something new. By next season, he needs to be established as a really good NHLer who just has to work out minor quirks in his game.