He's learning to play Center in the NHL and teams have a season of footage of just how absurdly explosive he is so they give him a lot more space in transition to keep from getting burned. Last year MacK had Stastny and Duchene to center the toughest minutes so he started off on a sheltered third line with Parenteau and Jamie McGinn before joining the Landy-Stastny line as a RW who teams just weren't ready for. This year he has to Center tougher minutes, especially with Johnny Malkin being injured so often. Add in that Landeskog and O'Reilly have been awful this year, Iginla has been settling in, and his struggles shouldn't be a surprise. Plus his first few strides are just in a league of their own, so teams couldn't expect him to fly past people like he did. Now they know just how ridiculously good his burst is, and he's not playing as well or with as much confidence which is why he's not entirely a victim of his circumstances.
My biggest gripe isn't even his defense, it was dreadful last year and was going to need a lot of work whenever we transitioned him to C. No, my biggest complaint is that too often he'll fly into the zone with the puck and at speed, but instead of using that speed and his power to charge the net hard, he curls off to the outside. Letting the defense back off of him and play the pass to the trailer. Maybe if his shot was on he could still force the other team to focus on him when he curled away, but with his accuracy being so off this year that doesn't happen.
This year is about the avs core learning to deal with everything going wrong after everything went right for us last year until the last month of the season, and we'll be better in the long run for it. That said MacK isn't even the biggest disappointment on his line as he's at least still doing good things. Landeskog and O'Reilly on the otherhand have genuinely sucked for much of the season and didn't even look like themselves on the ice for the first 17 games, and even since then both still aren't close to where they have been in previous years Landy is much weaker on his skates and on the puck this year, while O'Reilly has been slower than he was as a rookie and his offensive-touch is missing.
EDIT:
I'd also like to point out that while there were certainly avs fans who drank the koolaid too hard last year and ignored both his poor defense and the reality that he would have many more struggles whenever he was going to have to carry the tougher burden of centering a top 6 line, a lot of the more over-the-top hype came from non-avs fans.
Still a crazy talent who should one day surpass duchene to be one of the league's most dangerous forwards, and no we shouldn't have done something silly like taken a lesser talent just because Seth Jones may have learned to play his offside well enough to partner EJ down the line. Seth Jones is going to be an excellent player, but his raw talent isn't as special as MacKinnon's, and when you pick first overall you take the biggest talent in the draft, even if we didn't have EJ, Barrie, Siemens, and now Bigras making inexperienced but talented D not our biggest need.