You can do this without dealing any of the top 4 guys, even Nylander whom the media is beating their war drums again on. But if you aren't going to do that, you will have to make more major changes to the rest of the squad to support that core. The Leafs have to get a lot better at winning the close games they're going to encounter in the playoffs, and holding the leads they do manage to earn. Accepting that their high powered guys aren't going to score all over the place and wear down playoff calibre defences, they need to change the parts of the team they're willing to change -- the goaltending(we're assuming), bottom 6 and least half the defence -- to compensate.
first, Freddy has to go. he shouldn't be hard to get rid of with 1 year left. Sign Lehner. or make a deal with somebody. but we can't go on with a player in that important of a position who can't do the job in important games. If you keep him, you are running the very real risk that you could do a ton of shit right with the rest of the team, and it all goes down the drain with one Andersen softie. That cannot happen.
On defence, there should be plenty of options. Petro may not be realistic, but they can certainly find two decent to solid top 4 dmen somewhere. Muzzin/Rielly with two legit replacements for Ceci and Barrie is certainly a solid enough top 4. Dermott and Holl, with Sandin/Liljegren challenging them for a spot, is similarly solid enough for pair 3 duty compared to what most teams have.
Right now our 3rd and 4th lines don't really serve a defined role. they don't score a lot, they don't hurt people, and they're not a strong defensive unit either. other than Clifford it's a bunch more of what we have on the top two lines for the most part. small pass first offensive types. Dubas needs to go back to the drawing board with the lower half of the team, making it much more complimentary to the guys we're committed to most capwise.
So next you replace your neither-fish-nor-fowl 3rd line(~$10m), doable with any of the 3 of Kappy/Kerf/AJ because they're on decent contracts with a decent skill level and could be a good fit on a different team. This is your major opportunity to inject some defensive ability and discipline that we lack in the forward group. Robertson is one solid candidate. while small, he will get stronger, is a natural goalscorer, and plays with the type of jam and effort we need. he also looks like a guy we can mold into a good defensive player...unlike several other guys, the will to do so appears to be there with this kid. But this line needs to do much more for us than it has been in an overall sense.
next is the fourth line. I think at this point it's obvious Gauthier can't help at this level, and honestly if Keefe won't play him he's done here. so we are once again lacking a capable 4th line centre/effective 4th line. someone with speed and defensive ability to check and control the middle of the ice should be gettable on the free agent market or in a non-major trade. or we try Brooks. Spezza wasn't bad, but I think some younger quicker legs will serve us better. Clifford was ok, but he doesn't make enough of a difference on the 4th line that I would give up a 2nd to keep him. Engvall to me really tailed off in the NHL after a promising start. He'll be fighting for his life for a roster spot this winter, methinks.