A deal has been agreed for IBF champion Joshua to face Klitschko, but the challenger and former unified title holder is unlikely to face Joshua if the WBA 'Super' belt is not on the line.
Hearn is still hopeful that the WBA will sanction a fight between two of the star names in the heavyweight division but regardless of their final decision, he has said that the Olympic gold medallist will return to the ring on the same date.
He told Sky Sports News: "With the hold-up from the WBA, we need a little more time, so regardless of if it's Klitschko or not, AJ will return on December 10 in Manchester.
"We are beginning discussions with other opponents now while crossing our fingers that Klitschko takes the fight, regardless of how many belts are on the line.
"In an ideal world, there would be as many belts as possible on the line, but I hope he takes up the challenge rather than a soft touch in Germany."
If Klitschko decides to fight another opponent, Joshua must face someone ranked in the top 15 of the IBF rankings, with Kubrat Pulev, Eric Molina or Jarrell Miller likely to top the list of possible opponents.