British light heavyweight Buatsi made a winning yet disappointing start to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a fight where the techniques never quite clicked, with frequent clinching marring the fight.
Two judges scored it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my best display but how do champions respond? They succeed," Buatsi stated in the ringside as he was met by negative reactions from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Various analysts felt Parker, thirty-one, merited the win but the contender endured the second setback of his 28-bout career.
The Champion Earns Victory in Slow Affair
The fighter's advancement has stalled in the past few years through contract problems and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a last chance - under recent support.
He began cautiously, with neither fighter prepared to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who operated with a solid defense to barely take the better rounds.
The crowd's patience diminished amid frequent holding, with Parker going down various instances during grappling.
Parker finished the stronger, but many rounds were challenging to assess. Although, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Motivates All-Manchester Battle
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester contest to secure the European light-heavyweight belt.
The venue grew hushed for a extremely emotional homage ahead of the contest as a bell ceremony sounded in memory of Manchester's own Hatton, who passed away at age 46 in recently.
It was the first major televised boxing event in the location since Hatton's passing. As the concluding sound ended, the silence transitioned to a chant of "there's only one Hatton".
The fighting inside the ropes equaled the sentiment. The eventual winner was floored by a significant blow directly to the face in the second round, getting up unsteadily to his footing on the count of eight.
But the red-colored attire of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was injured quickly, noticeable damage appearing under his face as the fighters exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur barely secured the decision with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were relentless and brave, all the qualities their hero the legend embodied.
