Ruiz caused a major upset when he defeated Joshua - making his debut in the United States - by technical knockout in the seventh round of the June 1 fight to claim the WBA, IBF and WBO titles.
A rematch is likely to take place in November or December, though a venue is yet to be decided.
Joshua's choice is Cardiff's Principality Stadium, but Ruiz has refused to fight in the UK and a neutral venue is expected to be confirmed.
However, Joshua insists he is paying little attention to the negotiations and is solely focused on regaining his titles.