Leafs defense is a bit more dependent on how the forwards develop in terms of their responsibilities supporting the puck and giving open options to their defense.
Matthews and Marner* and Nylander are really good at quick backchecks and stealing the puck, but if the other team gets set up in our zone, they're almost non-entities. Nylander is probably the strongest defensively of the three, and that's somewhat horrifying given how everyone believes he's the worst DZ player ever to wear the Leaf (erroneously).
Larger numbers = more shots against. For reference, the team-wide shots against Threat% is +8; Matthews is +16, Marner is +13.
What has changed on the positive for the Leafs, even after losing Gardiner, is that there is far more puck competence on the backend now. Rielly, Muzzin, Dermott, Barrie, Schmaltz, Holl are all pretty comfortable handling the puck for more than two seconds. I think Ceci is okay if you support him with short outlets; same with Marincin. All they need to do is get the puck cleanly to a forward.
*Don't give me this nonsense that Marner is better defensively. People only think that because Marner looks like he's working harder (hunched over waterbug). Similar to Kapanen, they're always chasing the puck that way because they're in the wrong place.