Because Whyte failed a voluntary drug test, Joshua’s rematch with Whyte at heavyweight was called off over the weekend.
Helenius of Finland, who has 32 victories and four defeats, fills in with less than a week’s notice.
Joshua remarked about his new adversary, “This wasn’t in the plan. “The victory has my full attention.”
In October, Deontay Wilder knocked Helenius out in the opening round of their major fight.
The 39-year-old declared that he was “a true Viking” who was prepared to take on any task at any time.
“This was not a chance I was going to pass up. I want to utilize it to my advantage.
Only fighting on Saturday, Helenius defeated Mika Mielonen in Finland by third-round technical knockout. Helenius has never sought a world championship.
The 6’6″ Helenius has also suffered defeats at the hands of Gerald Washington, Whyte, and Johann Duhaupas.
A victory would position Joshua for his much anticipated matchup with former WBC champion Deontay Wilder, which may occur early in the following year.
In April, Joshua overcame Jermaine Franklin in the O2 Arena in London, where he is unbeaten.
To replace Whyte, Joshua, 33, had discussions with many of the heavyweights already on his undercard, including American heavyweight Washington, but promoter Eddie Hearn said that many opponents had overpriced themselves.
Joshua, according to Hearn, reacted “calmly” to the news of the fight’s cancellation when Whyte’s test result was disclosed to Matchroom and the boxing authorities by the Voluntary Anti-Doping Association (Vada).
“A week before one of the year’s biggest battles, I was astounded and perplexed. I received an incredibly calm message from AJ, said Hearn.
“AJ wants to fight like hell. In camp, he appeared amazing. He has never looked so wonderful to me.
It was confirmed in July that Joshua and Whyte would square off in a rematch of their 2015 battle.
In their heated British title match eight years ago, which also took place at the O2, Joshua defeated Whyte by knockout in the seventh round to revenge a defeat to his adversary on the amateur circuit.