Joshua plans to regain the IBF, WBA and WBO belts from Ruiz Jr in the main event on December 7, live on Sky Sports Box Office, and Whyte’s return could add an intriguing subplot.
Joshua’s rival Whyte has not fought since outpointing Oscar Rivas in July.
He became the mandatory challenger to the WBC title with that result, but the governing body suspended that status after Whyte returned an “adverse finding” in a UK Anti-Doping test.
Whyte said that he won the fight against Rivas “fair and square”.
But the WBC have delayed his world title shot, saying: “If his situation is resolved, he will become the mandatory challenger for February 2021.”
The WBC belt will be defended by Deontay Wilder against Luis Ortiz on Sunday morning at 2am, live on Sky Sports Action. Wilder then plans another fight with Tyson Fury in 2020.
Whyte’s wait for a world title shot was criticised as “frustrating” and “ridiculous” by Hearn.
“The UKAD situation is frustrating, we’re still waiting for them to issue a statement,” Hearn said last month.