In my opinion, Dubas has two paths forward at this juncture.
First option: Least amount of action is soft-reboot the rebuild with Robertson, Sandin, Liljegren, and move forward with the core that's already in place and accept that they'll likely be extremely mediocre and likely still poor defensively while an extremely young defence would have to learn on the fly. This would be if Dubas and company look at this Columbus series through the numbers which show that the Leafs got beat by insane goaltending and by all rights should've scored more than 3 even strength goals in 5 games (I disagree with this assessment, at some Leafs need to shit or get off the pot and beat the hot goaltender). Leafs core is locked in for a few more years yet and is still young. Team also need cheap talent to fill out the roster and unfortunately drafts 2015-2017 look like they won't yield much in terms of cheap young talent so we're kind of screwed there. This is the path of least resistance and would take the longest amount of time. Who knows if Tavares and Muzzin will be able to play at a high enough level for as long as it takes for these players to develop.
Second Option: Leafs move on from one of their core members, one of Marner or Nylander, to free up cap space and acquire a legitimate top four right-handed defenseman. Who's available for trade I've no clue. And it doesn't have to be a trade for an expensive world-beater. Just a solid top four guy who can be relied upon. Trade would also open up room to improve the overall depth of the team as everyone not on that super Matthews line in game 5 contributed nothing in the loss. Thought our offensive depth was supposed to be a strength but I suppose not. Another option, Pietrangelo is a free agent after this season but who knows if he'd want to come here, though the freed cap space from a Nylander or Marner trade (a Marner one especially) would free up the space for him. Pietrangelo is 30 and that sounds old but it's right in line with the ages of Tavares, Muzzin, and Andersen so he'd fit the makeup of this team. This goes in line with a win-now approach which is where this team is supposed to be anyway given how they've conducted themselves off the ice over the past couple of years, but the on ice results tell a different story.