Right, if Larson is pulled mid-race they can’t use a relief driver. Not sure how recent that rule change is, but that didn’t use to be the case.
If the race doesn’t start at all and he leaves for Charlotte they can put a different driver in there, it just has to be someone who ran over the past week’s sessions, which is only Siegel at this point. The team had planned for that to be TK, but they’ll have to hope the series bends the rules a bit and lets him run for 30 laps or so on Thursday.
It is kinda unfortunate this wasn’t entirely fleshed out better beforehand. If the backup can’t be TK, I don’t know how interested Hendrick would be in running the car at all, since they’re the ones funding it after all. I doubt Rick, Jeff, or anyone at HendrickCars.com had any idea who Siegel was before Sunday. I could just see the entry being pulled, unless it’s delayed entirely to Monday and Larson can make it back.