Armstrong checked into a treatment centre to get help for substance abuse in September after an on-stage outburst at the iHeartRadio festival.
Cavallo has now told Rolling Stone that Armstrong's stint in rehab is "no joke".
"He's under medical supervision for a while to get him clean," he said.
Cavallo - who has previously produced Green Day albums - added that he has no idea when Armstrong will be able to leave the centre.
"His doctors are still evaluating," he said. "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 recently cancelled their appearance at the Voodoo Music + Arts Experience Festival due to Armstrong's time in rehab and will be replaced by Metallica.
Watch the video for Green Day's 'Troublemaker' below: