To be fair, the Leafs are presently the 2nd best team in the league from a GA/GP perspective, slightly ahead of the Hurricanes, and a good half a goal better than Pittsburgh who's 6th best. They are 17th when it comes to GF/GP.
That being said, the suggestion that the Leafs can upgrade by simply trading Engvall is incredibly flawed. If the Leafs get fully healthy, they will be $783k over the cap with a 22 man roster (13f, 7d). They can solve this short term by sending Liljegren to the AHL and running 6 D, but that is certainly not a sustainable solution. Even if they move Engvall, they do not have enough cap space to replace him with somebody making league minimum.
Ultimately, the Leafs do need to "figure something out" on the left side. Bunting's been as good as one could possibly ask for at $900k, but he's a passenger. If he's in your top 6, then it should result in a having a bigger force on the 3rd line. Kerfoot has been solid, but again, is more of a passenger who's biggest value is that he can be the #2C if somebody gets hurt. Ritchie has been a massive disappointment and squandered the potential career-making gift of playing in the Leafs top-6. Mikheyev hasn't played a game yet, and Engvall is more of a utility forward than a difference maker.
Realistically, the Leafs will wait until Mikheyev comes back to see what they have in him before making a move to upgrade the LW. If he doesn't work out, then you'll likely see him move to create the cap space to bring in somebody else. Maybe Dermott in favour of a more physical presence too, or Ritchie, but I suspect the Leafs will be looking for size if they move Ritchie.