Anthony Joshua will fight Robert Helenius at London's O2 Arena on Saturday as the replacement for Dillian Whyte.
Joshua's heavyweight rematch with Whyte was cancelled at the weekend after the latter failed a voluntary drug test.
Finland's Helenius, who has 32 wins and four losses, steps in at less than a week's notice.
"This wasn't in the script," Joshua said of his new opponent. "I am laser focused on the win."
Helenius' last big fight was a first-round knockout defeat by Deontay Wilder in October.
"I am a true Viking that is willing to face any challenge at a moment's notice," the 39-year-old said.
"This is not an opportunity I was going to let slip away. I plan to make the most of it."
Helenius, who has never challenged for a world title, only fought on Saturday, securing a third-round TKO win over Mika Mielonen in Finland.
As well as Wilder, the 6ft 6in Helenius has losses to Gerald Washington, Whyte and Johann Duhaupas.
A win would set Joshua up for his long-awaited fight with former WBC champion Deontay Wilder, which could happen early next year.
Joshua is undefeated at the O2 Arena in London and won at the venue in April when he outpointed Jermaine Franklin.
Joshua, 33, held talks with numerous heavyweights already on his undercard, including American heavyweight Washington, to replace Whyte, but promoter Eddie Hearn said many opponents priced themselves out of the running.
According to Hearn, Joshua reacted "calmly" to the news of the cancellation of the fight after the Voluntary Anti-Doping Association (Vada) informed promoters Matchroom and boxing authorities of Whyte's test result.
"With a week to go before one of the biggest fights of the year I was shocked and dumfounded. AJ sent me a message which was unbelievably calm," said Hearn.
"AJ is desperate to fight. He has looked unbelievable in camp. I have never seen him in such good shape."
The fight between Joshua and Whyte, a rematch of their 2015 bout, was announced in July.
Joshua knocked out Whyte in the seventh round of their heated British title fight eight years ago, which also took place at the O2, as he avenged a loss to his rival on the amateur circuit.
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