UK light heavyweight Buatsi made a winning yet unimpressive beginning to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a close 10-round fight at the Co-op Live Arena.
But the challenger found success, connecting with powerful clean blows in a fight where the styles never quite clicked, with constant grabbing disrupting the fight.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best showing but what defines champions? They succeed," Buatsi commented in the ring as he was greeted with boos from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers felt the challenger, 31, should have been awarded the victory but the contender endured the second defeat of his 28-match career.
The Champion Secures Victory in Slow Encounter
The fighter's advancement has stalled in recent times through management issues and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers ready to be aggressive in a tentative initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who worked behind a solid defense to barely take the cleaner periods.
The audience's tolerance decreased amid frequent holding, with Parker going down several times during grappling.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Hatton Memorial Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a excellent local rivalry battle to secure the continental belt.
The venue grew hushed for a deeply poignant tribute before the fight as 10 bells sounded in remembrance of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the premier televised boxing event in the city since Hatton's passing. As the concluding sound faded, the stillness gave way to a shout of "there's only one Hatton".
The action within the ring equaled the emotion. The eventual winner was knocked down by a powerful punch directly to the face in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up soon, significant bruising forming under his eye as the duo exchanged powerful shots.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor barely secured the verdict with scoring tallies of even score, close margin and 115-112, but each competitor were relentless and courageous, exactly what their idol the legend represented.
