"It's really unfortunate for Samu and Nick, but a great opportunity for Jake and Rob to come into the squad," said Cheika.
"Jake was involved in our camp in Canberra earlier this year and has really stepped up for the Waratahs in Super Rugby.
"He's another uncapped player, but it will be good to expose him to the test environment."
Horne, who will join Northampton next season, was initially overlooked along with other senior players Scott Fardy (Leinster) and Will Skelton (Saracens), who have also signed with overseas clubs.
The 27-year-old Horne has played 29 tests for Australia, and has been in and out of the Wallabies squad since making his debut in 2010.
"Rob has been a strong leader in our Wallabies environment and his experience will be invaluable in this relatively inexperienced squad," Cheika said.
"We really want to put a strong emphasis on leadership in this team and we have some young leaders in this squad who are continuing to develop."
The Wallabies assembled in Melbourne on Sunday evening, ahead of Saturday's clash with Fiji at AAMI Park.
While Australian rugby has endured a tumultuous Super Rugby season on and off the field, Cheika is confident the Wallabies can turn things around.
"We've got play with a certain attitude and it's not going to be perfect, but a certain attitude that says 'Australian rugby - we're here for the fight'," Cheika said.