With the current situation - after Barrie and Ceci leave - the Leafs' presumably have Rielly, Muzzin, Dermott, Holl, Sandin, Liljegren (Marincin and Rosen). There are several major problems with this. Probably too many to fix with in the current cap situation.
Experience, grit, meanness, the ability to clear the front of the net and break the cycle. They have very little of any of that.
There is also the ongoing partner for Rielly problem which will require the Leafs to find a 1RD and the salary that will come with that. I personally think that the organization plans to solve the Rielly problem shifting him to the right side and going back to Muzzin - Rielly. They worked well together. We didn't see them at all this season, but the Leafs had Barrie, Ceci and Holl on the right and they without injuries on the right side until after Rielly was already out. I have been planning to go back and watch those games in early 2019 when the two played together, but haven't had the chance.
That potentially creates:
Muzzin - Rielly
Dermott - Holl
Sandin - Liljegren
But that leaves two pairs with almost no experience. Generally when people talk about experience they talk about the need to have a cup winner or two on your team to help know what it takes to go all the way. Certainly important, and the Leafs have Muzzin on D (and Clifford on F if he re-signs).
But there is a lot more to experience. Those 6 D have a total of 1373 games of experience (Muzzin - 579, Rielly - 517, Dermott - 157, Holl - 81, Sandin - 28, Liljegren - 11).
If you look at the top-6 in terms of playoff ice time for the last 10 Cup champs the average number of regular season games experience their top-6 had was 2881 games played. The lowest - the 2012 Kings had 2202 games played. Experience matters. The Leafs' D doesn't have it.
Plenty of those cup winners had a D in their top-6 with minimal NHL experience. Some had two. But four? Most of those teams had someone on their bottom pairing with around 900 games or more, and the 2015 Hawks had Timonen and Rozsival as a bottom pairing with 1998 games experience between them.
The other problem with lack of experience is the difficulty in bringing young D into your lineup. The Leafs' D pipeline is almost all the same type of D - small puck movers - Sandin, Liljegren, Hollowell, Duzsak, Lindgren etc. Their D-core is not set up to successfully bring those D into the lineup.
Over the years I have looked at young D being brought into NHL lineups going back to 2007/08 (the first year naturalstattrick gives TOI of teammates) and the types of partners placed with young D is what everyone should expect: experienced, or big/mean, or experienced and big/mean.
For instance, the top two D in the calder vote in 2008 were Enstrom and Niskanen. The former was partnered with 34-year-old Havelid and the latter split his time between 37-year-old Zubov and 36-year-old Norstrom. The two D in the calder vote the following season were Doughty and Bogosian. The former was partnered with 37-year-old O'Donnell and the latter was partnered with 39-year-old Schneider (in both of those cases those extremely experienced D were specifically signed to partner with those 18-year old rookies).
I have little expectation, based on history, and having watched them this season, that Sandin/Liljegren, Sandin/Holl, Sandin/Dermott, Dermott/Liljegren, Rosen/Liljegren, Holl/Liljegren etc will be any more than a complete waste of development.
In my view that either means 2 relatively cheap signings or a signing and a trade. Only two of Dermott, Holl, Sandin, Liljegren should be in the lineup.
This should also be the case if they keep Muzzin-Holl together and looked for a 1RD to play with Rielly. Then only one of Dermott, Sandin, Liljegren should be on the third pairing and the other D should be someone with the experience to help their development.