LAS VEGAS, NV - SEPTEMBER 21: Frontman Billie Joe Armstrong of Green Day appears backstage during the 2012 iHeartRadio Music
Christopher Polk

Almost a month since we reported that Green Day frontman Billie Joe Armstrong, was headed to rehab after an outburst at the iHeartRadio Music Festival in Las Vegas,  Warner Bros. Records Chairman and Green Day producer Rob Cavallo says the singer is still under evaluation and the band may have to cancel more shows.

"His rehab is no joke," the record producer tells Rolling Stone. "He's under medical supervision for a while to get him clean."

Cavallo, who has produced Green Day’s Dookie (1994) and American Idiot (2004), says he isn't sure when Armstrong will return to the stage. "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."

Green Day was forced to bow out of the Voodoo Music Experience taking place October 26 - 28, with Metallica filling in for their headlining spot.

As of now, the band is still scheduled to start their tour on November 26 in Seattle, though we'll keep you updated if that changes.