Anthony Joshua vowed to “get the belts back” after calling his sensational heavyweight world-title defeat by Andy Ruiz Jr a “minor setback”.
Joshua, 29, was floored four times as Ruiz defied underdog status to win in seven rounds and deliver a true shock to the sport at Madison Square Garden.
He said: “It’s a test of character. I’ll see what is next and move on.”
Promoter Eddie Hearn said a rematch is a “must win” because Joshua’s options will be “nothing after that”.
A rematch clause was part of the contract and Hearn said he expected it to take place “in England in November or December”. Ruiz himself says he will take the contest at any time.
Joshua had a concussion test after falling to a first defeat of his 23-fight career against a man who took the contest at short notice when original opponent Jarrell Miller failed drugs tests.
After scoring a third-round knock down, he was floored twice before the end of the round and put down twice more in the seventh before the bout was waved off.
Sitting alongside Joshua at a post-fight news conference, Hearn said his fighter will want the repeat fixture more with “every day that passes”.
Joshua quietly responded: “I think that’s right”.
He added: “Winning is everything. I am not going to sit here and say ‘losing is fine’. Not in my book. But if you do happen to lose, adjust and bounce back. But I do not condone losing in any form. This is war, battle, so what’s next?”
Joshua, who lost the IBF, WBA and WBO world titles, was asked about potential fights with WBC world champion Deontay Wilder and Tyson Fury.
“I still want to compete with those men but a rematch with Andy Ruiz Jr is where we are at,” he said. “I’m still a champ and I will rectify it to get the belts back.”
Ruiz warned he will be “more motivated as a champion” and vowed to take the rematch at “any time” after turning the heavyweight division on its head.
In becoming Mexico’s first world heavyweight champion he now holds three of the four major belts, with Wilder in possession of the other.