UK light-heavyweight Buatsi earned a win yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a judges' verdict in a close 10-round contest at the boxing arena.
But Parker found success, delivering effective single shots in a fight where the approaches never quite clicked, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best showing but what defines champions? They win," the victor commented in the arena as he was met by disapproval from the audience.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Several boxing experts believed Parker, 31, merited the win but the British boxer experienced the second defeat of his 28-bout professional journey.
Buatsi Claims Triumph in Dull Affair
The fighter's advancement has slowed in recent years through management issues and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under recent support.
He began cautiously, with both competitors prepared to take risks in a calculated opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second fight at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to narrowly win the better stanzas.
The crowd's patience diminished amid regular grabbing, with the challenger going down multiple occasions during grappling.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Homage Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a excellent local rivalry battle to secure the continental belt.
The venue fell silent for a extremely emotional tribute ahead of the contest as 10 bells rang in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised boxing event in the location since the passing of Hatton. As the concluding sound disappeared, the silence became a chant of "there's just one Ricky Hatton".
The competition within the ring reflected the emotion. Arthur was sent to the canvas by a significant blow squarely on the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored attire of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was injured quickly, noticeable damage developing under his orbital area as the pair exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the action rarely dropped over 12 rounds.
Arthur edged the verdict with judges scorecards of draw, 115-113 and 115-112, but both fighters were determined and courageous, exactly what their inspiration Hatton stood for.
