The band was rumoured to be on the verge of a split earlier this year (14) due to Young's health troubles, but they vowed to press on after the musician decided to step down for a hiatus.
Johnson has now revealed his friend is in hospital, but he remains hopeful of a recovery.
He tells TeamRock Radio, "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.
"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. 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."
No further details about Young's health battle have been given.
Johnson goes on to reveal work on the band's new album has been completed after a number of studio sessions in Canada.
He adds, "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."