Their D-core was pretty devastated this off-season with top 4 regulars Krug and Chara parting ways. Very young and inexperienced defenseman filling their shoes. They still have probably the best of group of defensive forwards in the league with Bergeron and Marchand leading the way.
Still though--four of the six Rangers defensemen today are either rookies or 2nd year players (Miller, Fox, Lindgren, and Hajek). The only two with more than 85 total NHL games under their belts were Jack Johnson and Brenden Smith. The Rangers were missing their top pair veteran leader on defense (Trouba), their best scorer by a LARGE margin (Panarin), and two of their best young players (Chytil and Kakko). And they played their backup goalie.
I think it's safe to say that MANY Rangers fans weren't shocked by the final score--though we were a bit shocked that those two numbers weren't reversed. Even with a young D, Boston still has an established, veteran group of forwards.