The only way to improve the defense is to trade Letang at this point. I have a hard time seeing a young coach coming in here and telling Letang that he no longer will be playing 26 mins/game. That he is no longer the team's PPQB.
I have an even harder time seeing Sullivan realize this himself.
But those two things are absolutely critical for this team improving going forward. It would be like demoting Evgeni Malkin to the third line. Letang has been the 24-26 minute first D on this team for so long, it is too late to give him a lesser role in my opinion. Especially since he is coming off a good regular season and has the stamina for it. But just because he CAN play all those minutes and see time on the first PP unit and the second PK unit, does not mean he SHOULD.
I have been saying this for a long time but they need to completely redistribute the ice time (and the roles) on defense. Pettersson and Marino should become our new No. 1 pairing, going up against all the top players in the league. A NEW PP QB should be added and he should play next to Dumoulin, who limits errors and mistakes when he is right. I don't know why he was not right against Montreal. But for what he brings to the table, I think he should stay.
The third pairing should be brand new. Ruhwedel is a 7 and no more. Joseph could be given a shot but there has to be a backup plan in case he is not ready for the jump. I honestly don't know what to do with Johnson. Maybe a team with significant cap space would take him in a package with a Hornqvist, a McCann, a Bjugstad, a DeSmith for minimal return (like the Gudbranson trade, basically). He can't stay, though. I actually like Jack Johnson but there is no point in keeping him if we are going to overhaul the blueline (which is what we should do, for sure).
The other option would be to trade Letang and instead of adding a PPQB only, go after Pietrangelo as your new No. 1. Pietrangelo is a better all-around defenseman than Letang and would at the very least stabilize things back there. Then you can add a potential PP QB on the cheap to run the first unit and play third-pairing minutes. Someone like Erik Gustafsson (not necessarily him).
Either way, this team cannot improve its defense with Letang still here. Merely dumping Schultz and Johnson won't solve everything if Letang is still being used 26 mins/game, on the top PP unit and on the second PK unit.
Those minutes have to go to someone else and those players are not on the roster right now.