There's no worst scenario for the show than having to drop to a two couples final. So if Jade's injury doesn't prove as bad as first thought, but they tell her something like it's best she didn't dance at all before Thursday or Friday, I don't see why they couldn't just let her do the tango. She'd have had little more training than anyone else by then for that dance, and a couple of days only to learn a new one in its place would be pretty harsh. Finding some way to keep her in, even if she can't train all week, would be a lot better than having to reinstate someone or skip an elimination week to keep a three couples final.