Green Day singer Billie Joe Armstrong has been in rehab for substance abuse since his on-stage outburst at the iHeart Radio Music Festival last month. The band's longtime producer, Rob Cavallo, spoke with Rolling Stone.

"His rehab is no joke," says Warner Bros. Records Chairman Rob Cavallo. "He's under medical supervision for a while to get him clean."

So far, the band has canceled appearances including New Orleans' upcoming Voodoo Music and Arts Experience. They currently have tour dates scheduled beginning November 26th at Seattle's Paramount Theatre and wrapping in February, but it's still up in the air whether more dates will be canceled.

"His doctors are still evaluating," says Cavallo, who has produced major Green Day albums including 1994's Dookie and 2004's American Idiot. "All I can say is I don't know if we'll see him before or after Christmas. It's basically undetermined when he'll be back."

Earlier this week, Green Day bassist Mike Dirnt told BBC Radio 1 host Zane Lowe the pressure to promote three albums – ¡Uno!, ¡Dos! and ¡Tre! – put a heavy burden on the band. "With hindsight 20/20, it was a tremendous undertaking," Dirnt said. "Although we were having fun, we didn't take our nose out of the book for a long time. I think it catches up with you a little bit. We definitely just jumped off a moving train."

The bassist added Armstrong's Vegas incident wasn't a complete surprise. "There were signs of things hitting the fan," he said. "We hadn't slept in forever and Billie had definitely had the worst of it. He'd been going through his own struggles . . . We were there with him, but you can only handle things on your own . . . At the end of the day, when we got off the road, the most important thing was my friend's life."