Dubois who holds the IBF title, underwent a medical examination in Saudi Arabia and missed a scheduled press conference, prompting the decision to pull out. Details of his health status have not been released.
"If he is ill, I hope he gets well soon and I'm looking forward to Saturday and a great show," said Parker, who will now face last-minute replacement Congolese fighter Martin Bakole.
British-born Dubois took the interim belt in June by beating Filip Hrgovic and was upgraded to champion when Oleksandr Usyk vacated the title due to his rematch clause with Tyson Fury.
Dubois then defended his title in September with a knockout win over fellow Briton Anthony Joshua at Wembley.
Former world champion Parker claimed the World Boxing Organization (WBO) heavyweight title in 2016 by defeating Mexican Andy Ruiz. He lost the title to Joshua in 2018.