British light-heavyweight Buatsi earned a win yet unimpressive debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a tight 10-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a fight where the techniques failed to mesh, with constant grabbing spoiling the contest.
Two judges had it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," Buatsi stated in the ring as he was greeted with disapproval 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 pro success.
Several boxing experts believed Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Claims Win in Slow Affair
His career development has stalled in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to achieve expectations - possibly in what feels like a last chance - under recent support.
He opened tentatively, with neither fighter ready to overcommit in a tentative early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to barely take the better rounds.
The audience's tolerance diminished amid constant clinching, with the challenger going down various instances during clinches.
Parker ended stronger, but multiple periods were challenging to assess. Although, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Motivates Manchester Derby Contest
In the co-main event, Arthur overcame Rea by split-decision in a terrific Manchester vs Manchester battle to capture the continental title.
The stadium became quiet for a extremely emotional memorial before the fight as 10 bells rang in memory of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the concluding sound ended, the silence transitioned to a roar of "there's just one Ricky Hatton".
The fighting within the ring reflected the emotion. Arthur was floored by a powerful punch flush on the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was injured early, significant bruising developing under his eye as the duo landed hard punches.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the action rarely dropped over the full distance.
The victor barely secured the verdict with scoring tallies of draw, narrow lead and 115-112, but each competitor were persistent and full of heart, all the qualities their idol the legend embodied.
