The 'Oh Love' singer entered a treatment facility in September after an on-stage outburst at the iHeartRadio Music Festival in Las Vegas.
Green Day bassist Dirnt has now admitted that it has been quite difficult for Armstrong's rehab stay to coincide with the release of new album Uno!.
"Billie doesn't take things lightly and he wouldn't take rehab lightly, either," Dirnt told Kerrang. "He's in the middle of some really big things, but we're going to pull through."
He continued: "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."
Warner Bros Records chairman Rob Cavallo disclosed this week that Armstrong could be in rehab through Christmas in order to fully deal with his issues.
Armstrong's rehab stint forced Green Day to cancel a headlining slot at the Voodoo Music + Arts Experience festival in New Orleans.
They have been replaced for the festival by friends Metallica.
Watch Green Day's 'Oh Love' music video below: