If we don't have Harley and Heiskanen but everyone else on this d-core, yes it would absolutely be a hail Mary.
It doesn't have to be Karlsson specifically. But we badly need an actual top 4 RD, one who doesn't suck with the puck on their stick would be a plus.
Based on Nill's response it means if they are adding another defenseman it's more bottom pairing fodder.
We can't go into the PO and realistic expect to win with 5 bottom pairing d-men to rotate through.
(Lyubushkin, Ceci, Bichsel, Smith, Dumba).
Adding a 6th (Petry, Dumoulin, Trouba, Jokiharju, Clifton, Martinez, Murphy, Ristolainen, Johnson, Savard, Ruuta, etc) doesn't do anything either.
I don't particularly want Karlsson, but there is no Top4 guys available(that we know of) that don't have high salaries and term.
Of the ones rumored to be available he should be the cheapest to acquire and has the shortest term.
You can insulate him with Lindell, much like Klingberg was, to cover up some of his warts.