British light-heavyweight Buatsi made a winning yet underwhelming start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a split decision in a competitive 10-round fight at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a contest where the approaches didn't complement each other, with excessive holding marring the fight.
Two judges judged it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest display but how do champions respond? They win," Buatsi stated in the ring as he was faced negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers believed Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
Buatsi Claims Win in Slow Encounter
His career development has stalled in recent years through contract problems and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to achieve expectations - possibly in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers prepared to be aggressive in a tentative early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who operated with a solid defense to narrowly win the better rounds.
The audience's tolerance wore thin because of regular grabbing, with the challenger going down various instances during clinches.
Parker ended stronger, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Inspires All-Manchester Battle
In the supporting bout, Arthur beat Bradley Rea by close verdict in a excellent all-Manchester fight to win the continental title.
The arena fell silent for a deeply poignant tribute ahead of the contest as ten bells sounded in remembrance of local hero Hatton, who succumbed at age 46 in September.
It was the premier televised fight night in the urban center since Hatton's death. As the final chime disappeared, the silence became a shout of "there's just one Hatton".
The competition inside the ropes equaled the sentiment. Arthur was knocked down by a powerful punch flush on the chin in the second round, getting up unsteadily to his position on the eight count.
But the red-colored equipment of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was marked up quickly, noticeable damage forming under his face as the duo traded punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur narrowly won the result with judges scorecards of even score, 115-113 and clearer margin, but both fighters were relentless and full of heart, exactly what their hero the legend embodied.
