The unified world heavyweight champion, 29, has not shared any fight details on social media since the 7 December rematch was announced on Friday.
His trainer Manny Robles says “there’s a reason why” Ruiz has kept silent.
Hearn, who promotes Joshua, said: “Both fighters signed for this fight. The governing bodies have been informed.”
At a London news conference where neither Joshua or Ruiz were present, Hearn said the rematch will take place in an open-air stadium in Diriyah, on the outskirts of Riyadh.
Omar Khalil, working for the SAudi agency delivering the event, said fans who purchase a ticket will automatically be issued a visa to visit the country.
More to follow.