British light-heavyweight Buatsi earned a win yet unimpressive start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a close ten-round bout at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a contest where the approaches never quite clicked, with frequent clinching disrupting the contest.
The officials judged it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They succeed," the victor said in the ringside as he was greeted with boos from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Various analysts believed Parker, thirty-one, merited the win but the British boxer experienced the second defeat of his 28-bout career.
The Champion Earns Triumph in Dull Encounter
The fighter's advancement has plateaued in recent times through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers willing to overcommit in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had kept it close, creating difficulties for Buatsi, who operated with a solid defense to narrowly win the more effective rounds.
The spectators' acceptance decreased during regular grabbing, with the challenger slipping to the canvas several times during grappling.
The underdog closed well, but many rounds were hard to judge. However, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Contest
In the co-main event, Arthur defeated Bradley Rea by close verdict in a excellent local rivalry battle to win the continental championship.
The arena fell silent for a extremely emotional tribute before the fight as ten bells rang in honor of Manchester's own Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime faded, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The competition during the bout reflected the emotion. The eventual winner was floored by a left hook squarely on the face in the initial stage, getting up unsteadily to his footing on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up quickly, significant bruising developing under his face as the pair exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor edged the verdict with official scores of even score, narrow lead and clearer margin, but both fighters were determined and courageous, all the qualities their idol the legend stood for.
