He has a new album to promote, touring obligations to plan for, and a stint in rehab to get through, but that wasn't enough to keep Green Day's Billie Joe Armstrong from kicking off his stint as a mentor on NBC's 'The Voice' last night (Oct. 8).

Armstrong was on hand for the first 'battle round' episode of the season, in which contestants from each of the teams (led by Blake Shelton, Christina Aguilera, Cee Lo Green, and Maroon 5's Adam Levine) duke it out in the ring with competing versions of songs -- like, for instance, Kansas' 'Carry on Wayward Son' or the Police's 'Message in a Bottle.'

Or Sublime's 'Santeria,' which contestants Bryan Keith and Collin McLoughlin performed, setting up the first "steal" in the history of 'The Voice' when both Shelton and Green tried to poach McLoughlin from Levine. (He ended up going with Shelton.)

Meanwhile, Armstrong popped in for some words of praise from Aguilera, who called him "amazing" and "a true rock star through and though" before telling the cameras, "I'm just so excited to have his knowledge and background come to the table." Armstrong was on hand for rehearsals of 'Message in a Bottle,' telling contestants De'Borah and Nelly's Echo that "Even the mistakes you're making sound amazing. I think this could be the best performance of the whole show."

Was Armstrong right? Check out the episode, embedded below, and let us know what you think.