World heavyweight champion Joshua (22-0) was slated to meet Jarrell Miller for his United States debut on June 1 at Madison Square Garden, but the American's failed drug tests meant the fight was scrapped.
Ruiz (32-1) is expected to be confirmed as the replacement to battle for Joshua's WBA, IBF and WBO world titles.
The California-born 29-year-old fought for a heavyweight strap against Joseph Parker in December 2016 and Hearn, who described a deal for the bout as "close", is sure fans will not be disappointed.
Speaking to Fighthype.com, he said: "He will entertain you, give it his all.
"Even if it means getting sensationally knocked out, he will rather do that than not do it.
— Anthony Joshua (@anthonyfjoshua) April 25, 2019
"I couldn't afford a guy who I thought may not be good enough or game enough to entertain the fans.
"I had to pick a guy in my heart who I thought will do everything to become world heavyweight champion at the Garden."
The unbeaten Joshua has been frustrated in his pursuit of a unification bout with WBC strap holder Deontay Wilder and has not been in the ring since knocking out Alexander Povetkin last September.