John Legend at the Greek Theatre

Before he found fame as an R&B; singer, John Legend labored briefly as a management consultant, and Tuesday at the Greek Theatre he put that business experience to work, waiting until he was a quarter of the way into his 90-minute show to make an offer he’d already fulfilled.

“Y’all wanna hear some love songs?” he asked. Of course we did; Legend has no other kind.

Over three albums he’s released since 2004, this 30-year-old Ohio native has come to fill a niche in the contemporary R&B; scene -- further evidence of his strategic mind. Unlike Maxwell, he doesn’t preach about the psycho-spiritual properties of sex, nor is he a Ne-Yo-style nice guy.

Rather, Legend subscribes to the Marvin Gaye school of thought: For him, love is an all-purpose vehicle of change, an antidote to the various “troubles in the world today,” as he put it Tuesday while introducing his song “Slow Dance.”

Like most pop sociologists, Legend’s not necessarily one for detailed solutions. “The news said the sky is falling, but it’s all right,” he sang in “Quickly.” Later, he advised turning down the television and turning up the John Legend as a means of coping with the prospect of global warming. If only Al Gore had thought of that.

At his best, though, Legend musters an infectious spirit that gives his dubious prescriptions real-world weight. He hit that sweet spot intermittently at the Greek, where he was backed by a 10-piece band that included three horn players and three backing vocalists. (Legend was to play the same venue Wednesday night.)

“Let’s not talk about the war,” he begged a lover in “Slow Dance,” then summoned a nimble neo-Motown groove that offered plenty of alternate stimulation. He transformed “P.D.A. (We Just Don’t Care)” into an up-tempo disco-soul jam, dropping in bits of Roberta Flack’s “Feel Like Makin’ Love.” And for “No Other Love,” a reggae-laced tune from last year’s “Evolver,” he welcomed the English singer Estelle onstage for a potent duet.

At one point Legend cleverly fused his song “It’s Over” with an affectionate cover of Michael Jackson’s “Remember the Time.” The invention was met with cheers, yet the comparison didn’t do the hard-working Legend any favors: Here was a guy thoroughly unacquainted with Jackson’s effortless ideal.

Indeed, Legend was least convincing when he tried the hardest, as in “Alright,” which had an over-muscled rap-rock clumsiness, and “I Can Change,” where he made a play at hip-hop currency by having his horn section quote Lil Wayne’s “Lollipop.” The result came closer to wedding-band territory than Legend likely intended.

In several stripped-down ballads, too, the singer overdosed on earnestness, nowhere more so than in “Good Morning,” for which he was joined by India.Arie, who opened Tuesday’s concert. The two harmonized skillfully, but proceeded with little regard for the considerable dangers of New Age treacle.

For an encore, Legend returned to the stage by himself and sang a powerful solo-piano version of “Ordinary People,” his first big hit. The presentation was simple and the message direct: “Maybe we’ll live and learn/Maybe we’ll crash and burn.”

It wasn’t much of a business plan, but it had the texture of life.

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