Anthony Joshua makes his American debut on June 1 against Ruiz, after Jarrell Miller failed a string of drugs tests.
Hearn and Matchroom Sport immediately put the feelers out for a potential replacement, as Joshua was keen to ensure the fight still went ahead as planned.
Miller was denied a boxing licence from the New York State Athletic Commission, and Hearn offered the fight to Ruiz.
Joshua’s IBF, WBA and WBO world titles will all be on the line, and for Ruiz the fight presents a golden opportunity to stamp his authority on the division.
Ruiz can become the first Mexican heavyweight world champion in history, and Hearn believes that this will be an added incentive for the underdog.
“I’m a little bit scared,” Hearn said, via Boxrec. “And I want to be scared.
“This is a new test for Anthony Joshua and now he has to fight a guy with a big set of cojones who will let his hands go, and show his entire heart.
“The fighters who talked about stepping up, my first thought was, ‘Who is it going to be?’ My concern was to make a real fight on June 1.
Anthony Joshua will take on Andy Ruiz on June 1(Image: GETTY)
Anthony Joshua was set to face Jarrell Miller at Madison Square Garden(Image: GETTY)
“I’m a fan. I didn’t want to bring Anthony Joshua to Madison Square Garden knowing we can’t lose.”
The fight was originally offered to Luis Ortiz after disgraced fighter Miller was forced to withdraw, but the Cuban demanded too much money for a deal to be made.
Instead, 29-year-old Ruiz was given the shot, and Hearn believes that he poses a real threat to Joshua.
“A couple guys reached out, but when I heard this man [Ruiz], I thought, ‘This guy really wants it. He thinks he can win. This is a tough fight. This kid can fight. This guy has Mexico behind him. This is a no-brainer,’” he continued.
Andy Ruiz Jr can become Mexico’s first ever heavyweight world champion(Image: GETTY)
“Andy Ruiz is not carved out of stone, but once this begins, this a real credible fight.
“There’s going to be a lot of British fans, a lot of Americans and a lot of Mexicans [in Madison Square Garden] because he has the chance to become the first Mexican heavyweight champion, and that’s an opportunity that may never happen again.
“If he wins this fight, he’ll be a national hero.”