Anthony Joshua relinquished his WBA (Super), WBO, IBF and IBO heavyweight titles to Ruiz Jr in front of a packed crowd at Madison Square Garden early on Sunday morning.
Joshua suffered a devastating seventh-round knockout loss to the Mexican-born American, who was a late replacement for Jarrell Miller.
Immediately after Joshua’s maiden loss, Hearn revealed he’d implemented a rematch clause in their deal with Ruiz, who stopped AJ on six weeks notice.
And on Tuesday afternoon, the Matchroom Boxing sport chief revealed Team Joshua have decided to exercise the clause.
“After meetings with AJ, Rob Mc and the management team in NY, we have today triggered the contracted rematch clause with Andy Ruiz Jnr,” Hearn wrote on Twitter.
“The fight will take place in Nov/Dec at a venue to be confirmed shortly.
Ruiz vs Joshua II is expected to take place in the United Kingdom, despite the fact latter was emphatically dethroned in the States.
Wembley Stadium and the Principality Stadium in Cardiff are the front runners to host the rematch, which will be the biggest fight of Joshua’s career so far.
Joshua was dropped a staggering four times before referee Mike Griffin mercifully called a halt to proceedings in the seventh stanza.
Plenty of questions have been asked about the Olympian’s preparations to face ‘The Destroyer’ in the days following the 29-year-old’s stunning victory, the biggest upset in boxing history since James ‘Buster’ Douglas beat Mike Tyson in 1990.
It has even been suggested Joshua was knocked out in sparring before the bout and that he suffered a panic attack moments before he was scheduled to make his ring walk.
Those notions have been emphatically dismissed by Hearn, who is adamant his man faced Ruiz in tip-top condition.
He told the Daily Telegraph: “Absolutely there is no truth that he was badly hurt in sparring, there were no health concerns going into the fight.
“The biggest concern was a stye on his eye.
“He’s putting on a brave face after the defeat, but he is already starting to think about getting revenge.
“The rematch is a huge fight and there’s a lot of pressure on AJ to win it.”