British light heavyweight Buatsi made a winning yet underwhelming start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round bout at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with excessive holding marring the contest.
Two judges had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They win," the victor said in the ringside as he was faced negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers thought Parker, thirty-one, deserved the win but the contender suffered the second defeat of his 28-bout career.
Buatsi Earns Win in Uneventful Match
The fighter's advancement has stalled in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to edge the cleaner periods.
The audience's tolerance wore thin because of frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
The underdog finished the stronger, but many rounds were difficult to score. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Inspires All-Manchester Fight
In the co-main event, Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester contest to win the European light heavyweight title.
The stadium fell silent for a profoundly moving tribute prior to the bout as ten bells rang in honor of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast fight night in the city since Hatton's death. As the final chime ended, the stillness became a shout of "there's just one Hatton".
The competition within the ring reflected the sentiment. Arthur was sent to the canvas by a powerful punch directly to the jaw in the early round, standing uncertainly to his footing on the eight count.
But the traditional red attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was injured quickly, noticeable damage forming under his eye as the fighters exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over 12 rounds.
Arthur edged the result with judges scorecards of even score, close margin and wider gap, but the two athletes were relentless and full of heart, exactly what their idol Hatton embodied.
