Ruiz produced one of heavyweight boxing's biggest shocks to take the Briton's IBF, WBO and WBA world titles.
"There's so much pressure on the rematch, as if he loses that second fight then his career is not in a great place," said Hearn.
A rematch clause was part of the contract for the first bout on 1 June.
Joshua, 29, was floored four times as USA-born Mexican Ruiz won in seven rounds at Madison Square Garden in New York.
The British London 2012 Olympic champion had a concussion test after the first defeat of his 23-fight career.
"At the moment, he's lost his world title belts and he's got a chance to win them back," Hearn said on his BBC podcast No Passion, No Point.
"If he was to lose that second fight, it's not career-ending but it starts to make you think about your career - and that's long before we expect that moment to happen.
"The rematch is by no means a forgone conclusion."
Ruiz, also 29, took the fight at short notice after Joshua's original opponent, Jarrell Miller, failed drug tests.
After the bout, the new champion said he would take a rematch at any time.
"Ruiz's confidence is going to go through the roof; AJ's confidence probably isn't going to be the same," Hearn added.
"I think he was probably too overconfident.
"What we saw was one of the biggest heavyweight shocks of all time. But we also saw an amazing fight, and now it's down to Anthony."