Eddie Hearn, promoter for Anthony Joshua, says his boxer will need to do a lot more in order to overcome Oleksandr Usyk in the rematch.
On Saturday night at Tottenham Hotspur Stadium, Usyk impressively outboxed Joshua over twelve rounds to capture the WBO, IBF, IBO, WBA heavyweight titles.
Joshua was unable to make the necessary adjustments, with Usyk landing accurate punches throughout the fight.
The British star's right eye began to swell up in the late stages of the contest, as Usyk poured on the punches in the championship rounds.
Joshua was in serious trouble as the twelfth and final round was coming to a close.
Hearn is obviously gutted, with unexpected loss creating an obvious delay to a potential unification showdown with the winner of next month's WBC title clash between Tyson Fury and Deontay Wilder.
There is a rematch clause and Hearn expects Joshua to exercise his contractual right to an immediate return match.
"He will have to make fundamental changes, he will have to box differently," Hearn told BBC Radio 5 Live.
"I can't see AJ taking a ten round fight, he boxes twice a year and maybe that is something we will have to look at.
"He is going to go away and all he will do is think about Oleksandr Usyk. He will be thinking: 'How can I beat him, how can I improve?' He is obsessed with improving.
"I just feel there is so much more Anthony Joshua in that fight but he will have to do a lot more in the rematch to be victorious."
The rematch with Usyk will likely come off in the first half of 2022.