Anthony Joshua will now face Robert Helenius at the O2 Arena in London on Saturday.
The two-time world heavyweight champion had been scheduled to fight old rival Dillian Whyte in a rematch of their 2015 clash at the same venue.
However, it emerged on Saturday that Whyte had provided an 'adverse analytical finding as part of a random anti-doping protocol' a finding that Whyte strenuously denies.
As it stands, the substance that Whyte tested positive for remains unclear, yet Matchroom Sport were in a position where they faced cancelling their second sold-out O2 crowd in less than 12 months if they could not find a replacement opponent.
While clashes with Dereck Chisora or Gerald Washington, who fight on the undercard, were rumoured, Joshua will instead square off against Helenius.
Heavy Duty at the O2 this Saturday 💣 No PPV - all part of your @daznboxing subscription. Let's GO 🔥 pic.twitter.com/jXERV7hDZn
— Eddie Hearn (@EddieHearn) August 8, 2023
At short notice, the Finn is a credible opponent as the 39-year-old holds a record of 32 wins and four defeats from 36 outings in the professional ranks.
That said, the most recent of those losses was a first-round knockout against Deontay Wilder, who Joshua is planning on facing if he can prevail at the weekend.
Helenius only made his return after a 10-month absence on Saturday, easing to a third-round win over compatriot Mika Mielonen in their homeland.
Speaking on the newly-scheduled fight, Joshua said: "This wasn't in the script. I respect Helenius and may I say, I respect any male or female who steps into the ring.
© Reuters
"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. May the best man win."
Commenting to Matchroom Sport's official website, Helenius added: "I am excited about fighting Anthony Joshua on August 12. I am a true Viking that is willing to face any challenge at a moments notice.
"This is not an opportunity I was going to let slip away. I plan to make the most of it!."
Whyte faced Helenius in Cardiff back in 2017, overcoming a slow start to record a resounding points win.
