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.
Anthony Joshua has vowed to get his 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.