My guess is that someone has to go down to make room and because Darlings contract is basically claim proof, he's temporarily getting moved off the roster until they figure something out. At this point, it probably wouldn't matter too much if they dropped Darling or Mrazek, but Mrazek would likely get claimed by one of the handful of teams looking for help in net... Except for the Flyers, since he was garbage there.
In a backwards way, the Canes should be thanking Darling for getting hurt so they had to go get McBackup... Otherwise a Darling/Mrazek tanden would've been a nightmare.