British light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a competitive ten-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective clean blows in a match where the techniques failed to mesh, with excessive holding spoiling the contest.
Two judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best showing but what defines champions? They succeed," Buatsi commented in the ringside as he was greeted with negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in February - to secure a 20th pro success.
Several boxing experts believed Parker, thirty-one, should have been awarded the win but the contender endured the second defeat of his 28-bout career.
Buatsi Secures Victory in Dull Affair
His career development has slowed in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who worked behind a tight guard to narrowly win the better rounds.
The crowd's patience wore thin during frequent holding, with Parker falling to the floor various instances during close exchanges.
Parker closed well, but several stanzas were difficult to score. Nevertheless, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a excellent local rivalry contest to secure the continental championship.
The arena grew hushed for a deeply poignant tribute ahead of the contest as 10 bells rang in honor of local hero Ricky Hatton, who died at age 46 in recently.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the last bell disappeared, the quiet became a roar of "there's just one Ricky Hatton".
The action within the ring reflected the emotion. Arthur was knocked down by a powerful punch squarely on the face in the early round, getting up unsteadily to his feet on the referee's count.
But the red-colored shorts of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was damaged soon, noticeable damage developing under his orbital area as the pair exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over the full distance.
Arthur narrowly won the decision with official scores of draw, close margin and wider gap, but both fighters were persistent and brave, all the qualities their hero Hatton represented.
