No Number-One Defenseman
Every Leafs defenseman is playing too high in the lineup.
Morgan Rielly is their #1 but he's really more of a two or a three. OEL has been getting #1 minutes, but he's really more of a 5 or a 6 and after a strong start he's fallen back to earth. Tanev is an elite #2 and is likely just as good as most top pairing defenders, but he doesn't score like one, the Leafs appear to be managing his minutes already, which can't be a good sign.
Jake McCabe is a good second pairing guy, but since they try to save Tanev and shelter Rielly, McCabe ends up being used like a top-pairing guy but he's just barely breaking even at 5v5 right now.
The Leafs blue-line is old, fragile and has very limited upside. Worst of all, perhaps, is that it's very slow. Outside of Morgan Rielly, the Leafs blue-line isn't exactly made up of speed merchants.
The Leafs blue-line needs to get younger, it needs to get more talented and it needs an injection of upside. The only way to do this outside of just randomly getting lucky is to trade for an elite defenseman.
In order to that, the Leafs are going to have to part with Mitch Marner or William Nylander.
I made a list of ten number-one defenseman the Leafs could target, but there is realistically only two or three that they could, and even then, only if they are willing to make an absolutely crazy blockbuster trade in the middle of the season.
Nothing about the Leafs overtly safe management style suggests that they would be into doing that, but if they aren't, it's going to be a very long season and Keth Pelley is going to be sending out some pink slips when it's over.