The new acting AG will take over. This is normal. (Sessions was recused from Russian matters because he helped Trump with his campaign; the new acting AG has no such potential conflicts). I don't think anyone has a feel for the Acting AG on this issue yet -- except presumably Trump. So Rosenstein losing his supervision of Mueller is normal.
Rosenstein resigning on the other hand is not.
Did Trump finally accept the resignation Rosenstein had reportedly offered? Did the new acting AG ask for a resignation? Did Trump?
It looks like Trump is moving fast following the elections.