"That defensive lineman..." You mean Shelton? I suspect that you may not fully understand what a nose tackle's job is in the Pats defensive scheme if you think he's "nothing special."
WRT linebacker prospects in this draft, it's extremely optimistic to think that any of those left on the board will be significant snap contributors this season. Langi, may well have a better chance. Sometimes, the guys you "need" just aren't there in a particular draft, no matter how much you wish they were.
Anyway, one of Casillas or Bowman would likely help more for 2018.