3. Call up Colton White
It's probably not everyone's favorite option, but calling up Colton White might be the option that comes with the least amount of red tape. White has spent the most time with the Devils, acting as the healthy scratch for almost all of December. He knows the team, the expectations from practice, what Sheldon Keefe is looking for, and he likely gained some chemistry with the Devils.
White is also acting as the Comets' top-line defenseman right now. He would play most similarly to Siegenthaler. He's nowhere near as good as him (no offense, but Siegenthaler is really good at defense), but he would be a seamless transition in the lineup.
After two season away from the organization, the Devils brought White back this offseason to add depth to the team. He's here in a situation like this where injuries are piling up and they need a body until they figure out what's next. Is a trade necessary? Can they reconfigure the lineup until Nemec is ready? Do they need a guy to play these next two games while Hatakka goes through a conditioning stint? White is your guy.
The Devils have some options. Nothing can replace Siegenthaler one for one from inside the Devils' organization, but they have the options to get them through while they wait to see how bad Siegenthaler's injury actually is.