British light-heavyweight Buatsi secured a victory yet unimpressive start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a contest where the techniques didn't complement each other, with excessive holding marring the contest.
The officials scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my best performance but what do champions do? They prevail," Buatsi said in the ringside as he was greeted with boos from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts thought Parker, thirty-one, merited the decision but the British boxer endured the second setback of his 28-bout career.
Buatsi Earns Triumph in Dull Encounter
The fighter's advancement has slowed in recent times through management issues and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He started carefully, with neither fighter prepared to overcommit in a tentative early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second bout at light-heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a tight guard to barely take the more effective stanzas.
The spectators' acceptance diminished during regular grabbing, with Parker going down multiple occasions during clinches.
Parker closed well, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Hatton Tribute Inspires All-Manchester Contest
In the supporting bout, Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester fight to win the European light heavyweight title.
The arena grew hushed for a deeply poignant homage ahead of the contest as 10 bells sounded in memory of the city's native Ricky Hatton, who passed away at age 46 in September.
It was the premier televised match card in the urban center since Hatton's death. As the last bell ended, the stillness gave way to a chant of "there's just one Ricky Hatton".
The fighting within the ring matched the feeling. Arthur was knocked down by a left hook directly to the chin in the early round, rising shakily to his footing on the count of eight.
But the red-colored equipment of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was marked up quickly, noticeable damage developing under his orbital area as the fighters traded punishing blows.
Rea, twenty-seven, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over the full distance.
Arthur edged the verdict with official scores of 114-114, 115-113 and wider gap, but both fighters were persistent and full of heart, exactly what their hero the legend stood for.
