British light-heavyweight Buatsi earned a win yet unimpressive start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a competitive 10-round bout at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a match where the techniques never quite clicked, with frequent clinching disrupting the contest.
The scoring judges had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my best display but how do champions respond? They win," the victor said in the ringside as he was greeted with boos from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Various analysts felt the challenger, 31, merited the decision but the contender experienced the second defeat of his 28-match professional journey.
Buatsi Earns Win in Dull Encounter
The fighter's advancement has slowed in recent times through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to realize his potential - maybe in what feels like a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to edge the more effective periods.
The crowd's patience decreased because of frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog ended stronger, but multiple periods were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a terrific Manchester vs Manchester contest to capture the European light-heavyweight championship.
The stadium fell silent for a deeply poignant tribute before the fight as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who died 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 transitioned to a shout of "there's just one Ricky Hatton".
The action inside the ropes reflected the emotion. The eventual winner was knocked down by a significant blow flush on the jaw in the early round, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up quickly, significant bruising forming under his eye as the pair landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over the full distance.
Arthur narrowly won the decision with judges scorecards of 114-114, 115-113 and 115-112, but both fighters were determined and brave, everything their hero the legend embodied.
