If RR replacement is not from within DOJ, then POTUS can appoint one of his other cabinet members, already confirmed by Senate, to be acting AAG.
I found three which have law degrees. Could be more
Dan Coates
Alex Azar
Alex Acosta
https://en.wikipedia.org/wiki/Alexander_Acosta
https://en.wikipedia.org/wiki/Alex_Azar
https://en.wikipedia.org/wiki/Dan_Coats