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.
AC/DC regrouped in Canada in May this year to record their next record, with Stevie Young filling in for his ill uncle.
"Stevie, Malcolm's nephew, was magnificent. But when you're recording (a new album) with this thing hanging over you and your work mate isn't well, it's difficult," Johnson said on Wednesday.
"But I'm sure he was rooting for us. He's such a strong man. He's a small guy but he's very strong. He's proud and he's very private, so we can't say too much. But fingers crossed he'll be back," he added.
No further details about Young's health battle have been given.
The band was rumoured to be on the verge of a split earlier this year due to Young's health troubles but they vowed to press on after the musician decided to step down for a hiatus.
"After forty years of life dedicated to AC/DC, guitarist and founding member Malcolm Youngis taking a break from the band due to ill health," the band said in a statement in April.
"Malcolm would like to thank the group's diehard legions of fans worldwide for their neverending love and support," the statement read.
"In light of this news, AC/DC asks that Malcolm and his family's privacy be respected during this time. The band will continue to make music."
Johnson also said in the interview that work on the band's new album has been completed after a number of studio sessions in Canada.
"We're done. I'm very excited and we've got some great songs. I wanted to call the album Man Down. But it's a bit negative and it was probably just straight from the heart. I like that," Johnson said.
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