There have always been two complementary sides of Paul Westerberg’s songwriting persona — the drunk ’n’ stoopid rawker of Replacements tunes like "Gary’s Got a Boner" and "Shootin’ Dirty Pool," and the sensitive singer-songwriterly artist responsible for "Aching To Be," "Unsatisfied," and "Here Comes a Regular." Perhaps the biggest mistake he made on his first few post-Replacements albums was to favor the latter without giving due time to the rocker who’d always offset his more maudlin moments. But under the guise of his alter ego Grandpa Boy, the Les Paul Jr.–wielding rock dude returned to CD on a single he released a couple years ago. And having freed himself of the major labels he never seemed comfortable with, this year he hooked up with the smaller Vagrant label and released two albums — a Grandpa Boy rawk album called Mono and a collection of his quieter, more melancholy material titled Stereo. It seemed that the Westerberg who’d been a potent hero to the post-punk generation of the ’80s was finally back, a little older, but still playing the anti-hero.

Surveying the stage at the Somerville Theatre a week ago last Friday night, where he played a solo show to a capacity crowd, you’d have had reason for concern. The second-hand sofa and chair — the only real stage props save for the four amps that littered the playing area — raised the specter of a seated Westerberg strumming his way through the mellower, more acoustic half of his extensive songbook. But the furniture was a prop: he emerged standing in front of a microphone stand with that wry grin on his face that I’ve always read as a portent of good things to come.

Indeed, without any band arrangements to follow, or even a set list that he felt tied to, he dispelled any fear that this was going to be a tame show as he tore into his 12-string acoustic guitar and began belting out "Waiting for Somebody," a track he wrote for the Singles soundtrack, and one of the better pop tunes he penned in the ’90s. Before long he’d visited just about every phase of his two-decade career, reaching back to his days with the Replacements for tracks like the gorgeous "Aching To Be" and the electric-guitar-powered "Valentine," cherry-picking the best tunes from his generally disappointing major-label solo discs ("Best Thing That Never Happened" from 1998’s Suicaine Gratifaction and "Knockin on Mine" from ’93’s 14 Songs), and offering a handful of new tunes from the Stereo/Mono outing.

As a performer and even a songwriter, Westerberg has always had an appealing self-depreciating streak; it goes all the way back to early Replacements classics like "I Will Dare" ("How young/smart are you?/How old/dumb am I?" is one of that song’s key refrains). That stance didn’t work quite so well when he was backed by the pro bands he toured with in the ’90s. But left to his own devices on a stage that was all his own, Westerberg not only wasn’t afraid to screw up, he wasn’t at all abashed about allowing the screw-ups to become part of the show.

So he took requests for a lot of Replacements songs he wasn’t quite prepared to play (but he did a fine version of "Can’t Hardly Wait," a tune you’d have to be a little crazy to try to pull off without at least a second-guitarist), he started songs only to abandon them mid-guitar stroke, he mumbled through verses he only half remembered, and he took it all in stride, not once showing even a glimmer of frustration or disappointment. In fact, he seemed happy to be free to make a mistake or two here or there, only to laugh it off and move on to the next tune.

By the end of the set, the stage was full of fans Westerberg had invited up to sit on the sofa as he nonchalantly tackled a trio of the Replacements’ finest tunes — "Swinging Party," "I Will Dare," and "Here Comes a Regular." Once again, he’d made a beautiful mess of things — the way the Replacements used to night after night. And you could tell from that smile that it felt as good for him as it did for the rest of us.