The Bruins will have plenty of flexibility while Marchand, McAvoy, and Grzelcyk are out on LTIR, but once the roster (hopefully) is fully healthy, the proverbial poop will hit the fan. The cap space does not work to field a full 23-man roster without any LTIR relief, even if the vets like Foligno, Nosek, and Wagner clear waivers and are demoted. With any luck, Foligno or Wagner will play well enough early on that they can either be traded or at least plucked on waivers, but their recent play and cap hits make the odds slim. Nosek has the most standalone value and is the most expendable name on the roster otherwise, so he likely will have to go.
This creates just enough cap room to have the kids run the show on the "fourth line". Even if some combination of Coyle, Smith, Zacha, Frederic, or anyone else becomes the real shutdown line as the third line, the reality is that the fourth line can and should be very young. With Studnicka and Steen no longer waiver-exempt and McLaughlin looking great at times late last year, they should all get a shot at regular play time. If Lysell, Beecher, or Merkulov are pushing for an opportunity as well, swap them out for McLaughlin.
If that plan works out, then by the deadline that ~$100K or so of cap space will have grown and McLaughlin's spot can go to a trade acquisition and then - cap space pending - the roster limit ends and McLaughlin and whoever else can fit can re-join the roster.
Realistically, whether the young fourth line works out spectacularly or not, forward depth is going to be the biggest need at the deadline if the roster is healthy (tons of depth on D and in net). Once the playoffs begin and the cap is no longer an issue, Foligno and Wagner are great assets to bring back into the fold. In the meantime though, a veteran forward and ideally a center will be the best target. It's not a great market, especially in the Bruins' price range, but old friends like Erik Haula or Marcus Johansson could be available, while Nick Bonino, Nick Bjugstad, Lars Eller, or Pius Suter could also be options. At wing, other old friends in Phil Kessel or Danton Heinen might be available, as could Max Domi, Andreas Johnsson, or Connor Brown. Worst case scenario, if trade costs are high or the cap space comes in too low, a mid-year contract for Joe Thornton would also be quite the story. However, if I am ideally looking for another play-making center to get the young kids going at an even higher level, I am going with Suter, who has played major roles on young Blackhawks and Red Wings teams, but shouldn't be too expensive to acquire.
You’re basically running this like the three injured players were on IR rather than LTIR. The money works out fine, there’s no need to move nosek, who is a good player, for nothing.