Despite Asuka saying that Kairi Sane was fine, the official storyline reason for why Sane is gone from the company is that she was given a career-ending injury at the hands of Bayley and Sasha Banks.
The idea is that Bayley attacked Sane so ruthlessly backstage that she could not return. This was supposed to be the angle they ran with, but Asuka cut a promo stating that she would be fine. WWE continued the storyline that she was injured anyway and ignored the promo by Asuka. The story was always meant to be that she was not fine which gave Asuka motive for revenge.