Joshua, 29, lost three of the four major world heavyweight titles to Ruiz in a huge shock in New York in June.
At London’s O2 Arena on Saturday – where Dillian Whyte beat Oscar Rivas – there was ringside talk of Joshua fighting again before facing Ruiz.
Hearn, Joshua’s promoter, said: “Do you think Joshua wants a warm-up? No.”
Hearn told 5 Live Boxing that Cardiff’s Principality Stadium was now the “front-runner” to host the rematch on 14 December, with a return to New York’s Madison Square Garden second in line for 29 November, a day after Thanksgiving in the US.
“It’s not even up for negotiation if he’s taking that fight or not,” added Hearn. “That fight is happening. Someone just asked me ‘do you think he should be taking this fight?’ I said ‘probably not’. In an ideal world probably you’d like a 10 rounder.
“If you’re a favourite for a fight for the heavyweight titles, you’re taking that fight. It is risky, dangerous, dramatic but it’s the biggest fight in world Boxing.”
Hearn said the thought of 80,000 fans watching the bout at Wales’ national stadium was “tempting” and the stadium’s capacity, coupled with the fact it has a roof to deal with the British winter, make it the only viable UK option for a fight of such magnitude.
Joshua, though, has previously expressed a desire to take on Ruiz, 29, at Madison Square Garden in order to land the win where his US debut was wrecked on 1 June.
‘Back to the drawing board’ – analysis
BBC Sport Boxing correspondent Mike Costello: “An American reporter asked Joshua if he would take a warm-up fight in the hours after the defeat at Madison Square Garden and Joshua snapped at him. There has probably been considered thought since then. I am convinced Ruiz is next. The danger for me is if there was to be an interim fight, that there becomes a danger fight if Joshua switched off.
“Something else struck me last week when a video was released on Joshua’s YouTube channel which told the story of the Ruiz setback.
“There was a striking image towards the end where he is in the dressing room after the fight and he is talking to his dad. His dad is saying to him very calmly, along the lines of ‘it’s time to go back to the drawing board’. Joshua agrees with him. I found that really interesting. At no time was there an excuse offered.”
5 Live Boxing analyst Steve Bunce: “There are two ways to go back to the drawing board. You can go back to basics and make sure you do the simple things well – your roadwork, your feet, your camp. The other way is to go back to the drawing board in your head and get a different head on. A different mindset going into camp for Joshua against Ruiz will be as important as the weights he lifts and how many rounds he spars.
“An alternative fight for Joshua isn’t great but a second defeat is even worse. That’s why I have changed my opinion whether a rematch makes sense straight away.
“I’m not sure the rematch makes sense. One or two of the tweets Joshua’s team have put out, talking of going back to the drawing board, suggest that what went wrong is not something they can put their finger on.”