Promoters Matchroom said on Tuesday the 39-year-old 'Nordic Nightmare', who fought in Finland at the weekend, will face the Briton at London's O2 with the undercard remaining as previously scheduled.
Helenius, who stopped compatriot Mika Mielonen in three rounds last Saturday, fought American former WBC world champion Deontay Wilder in New York last October and lost by first round knockout.
"This wasn't in the script," said Joshua, who lost his WBA, WBO, IBF and IBO belts to Ukraine's Oleksandr Usyk in September 2021.
"I am laser focused on the win. I can make steps forward to bigger and better things but the road map has a check point, Saturday night."
Whyte was removed after it was announced last Saturday he had returned "adverse analytical findings as part of a random anti-doping protocol".
Marchroom Sport chairman Eddie Hearn said Helenius was a good opponent.
"Yes, he lost to Deontay Wilder but that was by a shot that would have probably knocked out a horse. But he came to fight. He can really punch, he’s 6ft 8in and he won at the weekend," added the Briton.
"Ultimately there is criticism in whatever you do, but I actually think this is a tricky and dangerous fight."