IBF champion Joshua can add the WBA 'super' title with victory over Klitschko in Saturday's heavyweight unification clash, live on Sky Sports Box Office.
A record crowd of 90,000 will attend the fight, which will also be watched by a worldwide audience on television, and Hearn believes the unbeaten 27-year-old could soon be recognised as the most marketable figure in the sport.
"He becomes the biggest star in British sport and the biggest star in world boxing," Hearn told Sky Sports. "It's early in his career, it's his 19th fight, so masses of pressure."
Asked about Joshua's meteoric rise, Hearn said: "We talk about this 10-year plan, we're kind of having a legacy fight within the first couple of years. It's here, the timing is perfect.
"He could sell out any arena, against anyone, but he wants to be remembered for the big fights.
"This opportunity won't be there forever. You fight Klitschko now, or it never happens. He wants to be in those big fights, he wants to take on the very best."
Klitschko is standing in the way of ambitious plans for Joshua to dominate the top division and Hearn is fully aware of the threat posed by the former unified champion.
"He 100 per cent thinks he has got AJ's number, I'm convinced of that," said Hearn.
"I know he thinks he's dangerous, but he doesn't feel like AJ has the tools to overcome him.
"He might be wrong, but I want the fight to over in six rounds. I don't want AJ to go into seven or eight with this guy because he's experienced, he's strong."