The Mexican holds three world heavyweight titles following his stunning win over Joshua in New York in June, but is awaiting confirmation of their second fight, with Cardiff’s Principality Stadium leading the shortlist of locations.
Talks are ongoing with Ruiz Jr’s team, although Hearn has confirmed the California-based fighter will not receive home advantage in a return bout.
“I have been speaking to Ruiz’s team this week, keeping them up to speed with where we’re going,” Hearn told Sky Sports News.
“They don’t want to come to the UK, they never have done. I think they used the Dillian Whyte saga as a little bit of a scapegoat in that decision as well, and we have to take that into consideration.
“Contractually we know our position, but we want to make sure we get the fight done. We want to respect Andy Ruiz, but we’ll decide where the venue is.
“A neutral venue, he would be a lot happier with. I think I can rule out America, right now. Cardiff is still the frontrunner, it’s still the favourite.”
Unnamed venues remain in consideration for the rematch, which is due to be staged in November or December, and Joshua is already preparing to face Ruiz Jr again.
“There are some other interesting options that would be quite iconic internationally, which does involve a lot of aggravation logistically, but does involve a spectacle that would be quite unique,” said Hearn.
“I keep saying this, but in the next 48 hours Andy Ruiz’s team will be alerted to where this fight will take place.
“Joshua is working hard to make sure he’s ready to win those belts back.”