Wisniewski is out for 6 weeks and Duchene for 4. You can drop 2 of the 4 to pick up new players, but they may no longer be available when they come off IR. Or you can keep them on your bench and you just won't have an replacements.
I had Richards, Lehtonen and Letang out on IR at one time plus Chara and Skinner DTD. And then later I had Skinner, Letang and Lehtonen on IR with only 2 spots available. Both times I just opted to keep the players and have one less available bench player. It's tough with all the guys going out with concussion symptons because there is no definite timetable on when or IF they will return.