Billie Joe Armstrong, lead singer for Green Day, has been in rehab for substance abuse since last month following an onstage meltdown at the iHeartRadio Festival in Las Vegas. In a new interview, the band's longtime producer says that he is unsure of when Armstrong will be cleared to resume his career.

"His doctors are still evaluating," Rob Cavallo told Rolling Stone. "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."

The band is expected to begin their upcoming tour in support of their new album, '¡Uno!,' on Nov. 26 in Seattle. However, the statement by Cavallo, who is also the Chairman of Green Day's label, calls into question whether or not they will be able to honor all of the dates. They have already canceled their appearance at the Voodoo Music + Art Experience in New Orleans on Oct. 27. As a show of support, Metallica has been slated to replace them.

“Our good friends and fellow Bay Area residents Green Day had to unfortunately cancel their plans to be there, so we are more than happy to fill in for them in our own unique way,” said drummer Lars Ulrich. “We’re hoping we can fill those very large shoes and do them proud.”