Eddie Hearn, promoter for Anthony Joshua, says the boxer's career will be at stake in the rematch with Andy Ruiz.
On Saturday night, Joshua's world came crashing down when he was stopped by Andy Ruiz in one of the biggest upsets in boxing history.
Ruiz spoiled Joshua's United States debut, by dropping the British star four times for a seventh round TKO at Madison Square Garden in New York City.
The victory gave Ruiz's the WBO, IBO, IBF, WBA world heavyweight titles.
Joshua plans to execute an immediate rematch clause, with the second fight targeted for the fall in the UK.
Joshua is making no excuses, and promises to win back his world title.
"He's not the kind of guy to cry and sulk. He's going to go away, evaluate his performance and improve," Hearn said to BBC Five Live.
"Last night I saw I guy I knew wanted the belts back. This morning I saw a guy where that was the only thing on his mind, to become heavyweight champion of the world again. He had Ruiz in desperate trouble, went in to finish him recklessly and got caught by a shot on the top of the head that he never recovered from.
"I don't think he really knows a lot about the fight after the third round. Despite the fact that he kept getting up, I think he was concussed, I think he never recovered. If he was composed and a little bit more experienced I think he'd have taken him out in the third round but he paid a terrible price. The referee asked him if he wanted to continue, he said "yes". He asked him if he was alright to continue, he said "yes".
"Unfortunately what he didn't do was put his hands up because he was trying to hold onto the ropes to stand up. He would never quit. He kept getting up and up and would have done it all night until he was unconscious. Andy Ruiz deserves all the credit in the world. We knew this was going to be a tough fight. Although he doesn't look good in a pair of trunks, and that was why we were reluctant to take the fight, because we knew in the build up that people wouldn't give Andy Ruiz a chance to beat Joshua.
"Last night was one of the biggest upsets in heavyweight boxing history, well probably in boxing history and now the next chapter is, can he return? Can he win back his belts in what will be a much anticipated rematch? If AJ bounces back and defeats Ruiz in a rematch, he's a bigger star than he was prior to [Saturday] night. The rematch is so important to his career, we went to Ruiz's country so I expect the next one to take place in the UK."