UK light-heavyweight Buatsi earned a win yet underwhelming start to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective individual punches in a contest where the approaches never quite clicked, with constant grabbing spoiling the fight.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest display but what do champions do? They prevail," Buatsi stated in the arena as he was met by boos from the audience.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Various analysts believed the challenger, 31, deserved the decision but the contender experienced the second loss of his 28-bout career.
Buatsi Earns Victory in Uneventful Affair
His career development has slowed in the past few years through management issues and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now is positioned trying to achieve expectations - maybe in what seems like a last chance - under fresh management.
He started carefully, with both competitors ready to overcommit in a tentative initial rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to narrowly win the more effective stanzas.
The crowd's patience wore thin because of constant clinching, with Parker going down several times during clinches.
Parker finished the stronger, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a outstanding all-Manchester battle to capture the European light-heavyweight championship.
The venue grew hushed for a profoundly moving tribute before the fight as a bell ceremony tolled in honor of Manchester's own Hatton, who succumbed at age 46 in September.
It was the first major televised fight night in the urban center since Hatton's death. As the final chime disappeared, the silence became a chant of "only one Hatton".
The action inside the ropes equaled the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the chin in the initial stage, getting up unsteadily to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was injured quickly, noticeable damage developing under his eye as the pair exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity seldom decreased over the full distance.
The victor edged the result with judges scorecards of even score, close margin and 115-112, but each competitor were relentless and brave, exactly what their hero the legend stood for.
