Hearn confirmed on Saturday night that Joshua had triggered his rematch clause against the Ukrainian.
Usyk stunned Britain's Joshua last month, winning by unanimous decision to take the WBA, IBF and WBO heavyweight belts.
"He [Joshua] has got his head back in the game," Hearn said.
"He is training now and today we officially triggered the rematch for the Oleksandr Usyk fight which we will see early next spring.
"We're looking for him to become a three-time heavyweight world champion. I think realistically March is when you're likely to see the rematch."
