Johnson spoke of Young's health battles, reported earlier this year, during a recent interview with Classic Rock magazine to talk about the Australian rock band's 15th album.
The frontman said the album was now complete and said Young's absence from the recording studio had been missed, revealing his friend was in hospital.
"We miss Malcolm obviously. He's a fighter. He's in hospital but he's a fighter. We've got our fingers crossed that he'll get strong again," Johnson told Classic Rock.