I was mostly just replying to the "They're not supposed to be good this year" comment. I think that is more of a recalibration based on what has happened.
Hockey doesn't use WAR as much as baseball due to the nature of the sports, but surely you could at least eyeball the number of wins that you lose because of Marner. Let's say Marner was responsible for 7 wins above replacement last year. That's 14 points. So instead of 108, they get 94. That should still put them into a playoff spot, provided they get the same goaltending and relative performances from other members of the team. So even if Matthews played like he did last year, they still had a decent shot at a playoff spot. If Matthews rebounded, then you'd be even better off.
Now we are roughly a third through the season, and this team started on shaky ground, has rebounded a little bit, but that start has put them in a precarious position because they might not have the talent to pull themselves out of it. They are currently on pace for 87.2 points. That's another 3 - 4 wins you could attribute to the loss of Marner I suppose, but at that point you are saying that Marner was responsible for 10 - 11 wins of the Leafs 52 wins last year. I guess it's possible that he is that good, and he was responsible for 20% of the Leafs wins last year. To me that would say that Berube or Treliving are worse than we thought, and I already thought they weren't the right people for this team.