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."