He certainly had a backslide year. I think a LOT of it IS who he is partnered with a lion's share of the time. I don't think that's a myth at all... but yeah HE has also been shitty, the two things aren't mutually exclusive. His numbers would CERTAINLY be better if he wasn't stapled to Harpur so much, but they still wouldn't be good. Most dmen as young as him don't go from trying to do too much and cover more ground for defensively compromised partners to being smooth and confident when you get paired with a decent one. In fact most dmen as young as him haven't made the NHL yet. Kid is still only 21. We saw last year what he can do, and in spurts this year. He gets a nice run of games with a reliable NHL level partner and I think his game comes back.
And no this isn't a defense of his play this year, or an excuse, he's played badly, but it's not because he is unable to play better. He's another guy who can benefit from a good off season program.