UK light-heavyweight Buatsi secured a victory yet unimpressive debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a close ten-round fight at the boxing arena.
But Parker found success, connecting with powerful clean blows in a match where the approaches never quite clicked, with excessive holding spoiling the action.
The scoring judges judged it 96-94 to the winner, with the third a even score.
"It wasn't my finest display but what do champions do? They succeed," Buatsi stated in the ringside as he was met by boos from the crowd.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers thought Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Match
His career development has stalled in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to realize his potential - maybe in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers ready to overcommit in a calculated initial rounds.
Parker, 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 fight at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who operated with a solid defense to edge the better periods.
The spectators' acceptance wore thin because of regular grabbing, with Parker slipping to the canvas multiple occasions during grappling.
Parker finished the stronger, but multiple periods were hard to judge. Although, former world champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester contest to secure the continental championship.
The arena became quiet for a deeply poignant tribute before the fight as ten bells sounded in honor of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised match card in the location since Hatton's death. As the concluding sound disappeared, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The action during the bout equaled the feeling. Arthur was knocked down by a significant blow flush on the jaw in the initial stage, getting up unsteadily to his footing on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was damaged soon, noticeable damage appearing under his eye as the fighters landed punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the pace rarely dropped over the full distance.
The victor barely secured the verdict with scoring tallies of draw, close margin and 115-112, but both fighters were persistent and courageous, exactly what their hero the legend stood for.
