The group had previously announced in April that Young would be taking a break from the band and that it would be recording a new record without him. When AC/DC announced that it would be putting out the record, "Rock or Bust," this fall, they confirmed that Young would not be returning to the band, "due to the nature of Malcolm's condition." They did not go into the specifics of the illness.
Young's nephew, Stevie Young, played rhythm guitar in Malcolm's stead on "Rock or Bust." He will also be filling in for the elder Young on the group's upcoming world tour in 2015.
"We miss Malcolm, obviously," AC/DC frontman Brian Johnson told Classic Rock in July at a time when the band was optimistic about the guitarist's return. "He's a fighter. He's in (the) 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 with this thing hanging over you and your work mate isn't well, it's difficult. But I'm sure (Malcolm) was rooting for us."
"Rock or Bust" will feature 11 new AC/DC songs when it comes out on December 2. In September, the band began teasing the song "Play Ball" in ads for Major League Baseball's Postseason campaign on Turner Sports. (Turner Sports is a division of Time Warner, as is CNN.)
source: cnn.com By Kory Grow, Rolling Stone