British light heavyweight Buatsi earned a win yet underwhelming debut to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a tight 10-round fight at the Manchester venue.
But Parker found success, connecting with powerful single shots in a contest where the techniques never quite clicked, with constant grabbing disrupting the contest.
The officials had it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi stated in the ringside as he was greeted with negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts thought Parker, thirty-one, merited the win but the contender experienced the second loss of his 28-fight professional journey.
The Champion Claims Victory in Uneventful Match
Buatsi's progress has plateaued in recent times through promotional disputes and extended periods of lack of fights.
Earlier forecast as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a last chance - under fresh management.
He began cautiously, with neither fighter willing to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who worked behind a protective stance to edge the more effective rounds.
The audience's tolerance diminished during constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Hatton Homage Inspires Local Rivalry Battle
In the co-main event, Lyndon Arthur beat Rea by close verdict in a excellent all-Manchester contest to capture the European light-heavyweight championship.
The stadium grew hushed for a profoundly moving homage ahead of the contest as a bell ceremony sounded in memory of Manchester's own Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime disappeared, the silence gave way to a roar of "only one Hatton".
The action during the bout reflected the sentiment. Arthur was floored by a significant blow flush on the jaw in the second round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red shorts of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was damaged soon, noticeable damage developing under his eye as the duo traded punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over 12 rounds.
Arthur barely secured the result with scoring tallies of even score, narrow lead and wider gap, but both fighters were determined and courageous, everything their inspiration Hatton stood for.
