When Jason Isbell first joined the Drive-By Truckers — just after the release of their breakthrough Southern Rock Opera — he looked a little out of place. Baby-faced and younger than founding Truckers Patterson Hood and Mike Cooley, he seemed like a little brother to the other two, stepping out of their shadow to take a solo or sing lead on one of his songs.

Then came Decoration Day: the Truckers’ true coming out party, and the first indication of the band as the three-headed songwriting monster they had become. It kicked off a run of three terrific albums, God-only-knows how many marathon rock shows, and brought the band to a much wider audience. So when Isbell announced he was leaving the group this past winter, fans started to wonder how he and the band he left behind might fare.

With the Truckers playing to a sold out 9:30 Club this Friday, they’ve got quite a task ahead of themselves. For Isbell and his new group, the 400 Unit, are a force to be reckoned with. Where their first release, Sirens of the Ditch, took Isbell’s knack for evocative storytelling, twang, and guitar muscle and spun it into a polished pop record, their set Wednesday night at the Rock and Roll Hotel was simply roaring.

Two hours of blasting guitars kicked off with the bluesy, slow burn of “Down in a Hole,” before stepping up the pace with “Grown.” A cover of a Patterson Hood song (“The Assassin”) showed there were no hard feelings (Isbell later told folks to check out their show Friday), and Isbell tore into his old band’s material as the set continued. “Goddamn Lonely Love” lagged some at the beginning, but the full band pickup before the last chorus was crisp and rocking to bring the song home.