UK light-heavyweight Buatsi made a winning yet underwhelming beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the approaches failed to mesh, with constant grabbing disrupting the contest.
The scoring judges scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but what defines champions? They prevail," the victor stated in the arena as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, deserved the victory but the British boxer experienced the second defeat of his 28-match professional journey.
Buatsi Secures Victory in Dull Match
Buatsi's progress has stalled in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a last chance - under fresh management.
He started carefully, with both competitors willing to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who worked behind a solid defense to edge the cleaner periods.
The crowd's patience decreased amid frequent holding, with the challenger going down various instances during clinches.
Parker closed well, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just a single round.
Buatsi advances, but with the presence of power puncher Benavidez, and Russian champion Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester battle to secure the European light heavyweight belt.
The arena grew hushed for a profoundly moving homage prior to the bout as a bell ceremony tolled in remembrance of the city's native Hatton, who died aged 46 in last month.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound ended, the stillness gave way to a chant of "there's only one Hatton".
The competition within the ring matched the sentiment. Arthur was knocked down by a significant blow directly to the face in the second round, standing uncertainly to his feet on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was damaged soon, heavy swelling developing under his eye as the pair landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the intensity rarely dropped over twelve rounds.
The victor edged the verdict with official scores of draw, close margin and 115-112, but both fighters were relentless and courageous, everything their inspiration the legend stood for.
