"The reality is we have to put some faith in young blokes and hope they stand up".
Hilditch said off-spinner Nathan Hauritz was overlooked as the selectors wanted a slow bowler to turn the ball away from the bat in an England middle-order featuring right-handers Jonathan Trott, Kevin Pietersen, Ian Bell and Matt Prior.
Hilditch said selectors had no idea if Johnson's remedial work on his action in Perth this week with mentor Troy Cooley had worked, but said he could still make the XI.
"We are really keen on Mitchell coming back. Whether he is ready for Perth I don't know quite yet," Hilditch said.
"He is still a work in progress."