Armstrong checked into a treatment facility to seek help for unspecified substance abuse problems following last month's iHeartRadio Music Festival during which the musician ranted onstage because he thought his group's set was cut short to make way for other acts.
The band later said they had as much time as expected and apologized for Armstrong's behavior.
"Billie doesn't take things lightly and he wouldn't take rehab lightly, either," Dirnt told Kerrang! magazine. "He's in the middle of some really big things, but we're going to pull through. It's killing everyone. Green Day has never been a band that sits still, but sometimes you have to step back and let life go on rather than try to control everything."
MTV.com said the band doesn't expect to see Armstrong before Christmas as his doctors are still evaluating him.
Green Day has canceled numerous concerts while its front-man seeks help for his problems.