UK light heavyweight Buatsi earned a win yet disappointing beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close ten-round fight at the Manchester venue.
But Parker found success, connecting with powerful single shots in a fight where the approaches failed to mesh, with constant grabbing marring the action.
Two judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best display but what do champions do? They win," the victor said in the ringside as he was met by boos from the spectators.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts believed Parker, thirty-one, deserved the decision but the Derbyshire fighter suffered the second loss of his 28-fight career.
The Champion Secures Win in Uneventful Affair
The fighter's advancement has stalled in recent years through management issues and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He began cautiously, with both competitors prepared to overcommit in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to barely take the cleaner periods.
The audience's tolerance decreased amid constant clinching, with the challenger falling to the floor various instances during close exchanges.
The underdog closed well, but several stanzas were hard to judge. However, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Memorial Inspires Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding Manchester vs Manchester contest to win the continental belt.
The venue fell silent for a extremely emotional tribute prior to the bout as ten bells rang in remembrance of the city's native Ricky Hatton, who passed away at age 46 in September.
It was the first major televised fight night in the urban center since the passing of Hatton. As the final chime faded, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The competition within the ring equaled the sentiment. The eventual winner was knocked down by a significant blow squarely on the face in the second round, rising shakily to his footing on the count of eight.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured quickly, significant bruising developing under his orbital area as the pair landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the result with scoring tallies of even score, 115-113 and clearer margin, but both fighters were persistent and brave, all the qualities their idol Hatton stood for.
