Marchand and Eriksson consistently were on the PK. Marchand has been for years now. Krejci had to be rotated in, but it's not a position he hasn't filled in the past before. He would have helped on the PK, but I don't believe that Kelly is the difference in being outscored 7-1 in the final two games.
Seidenberg was still around the team plenty, practicing and skating every day, Chara still there (I don't believe that injuries affect leadership abilities?), Thornton, Lucic, Krejci, Bergeron, Boychuk, etc.
If the team doesn't have enough leadership to keep it together in the playoffs and give some sort of effort against Montreal, we've got some serious issues on that front. I don't believe that's the case at all though. It was an issue last year as well against Toronto, with Chara fully healthy, Ference in Boston, Kelly on the ice, Seidenberg on the top pair, etc.