UK light heavyweight Buatsi earned a win yet underwhelming start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a fight where the techniques didn't complement each other, with constant grabbing disrupting the action.
The scoring judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They succeed," the victor stated in the ringside as he was greeted with negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers thought Parker, 31, merited the victory but the Derbyshire fighter suffered the second defeat of his 28-bout career.
The Champion Earns Win in Dull Encounter
The fighter's advancement has plateaued in recent times through management issues and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter prepared to take risks in a calculated initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second appearance at the higher weight class.
By the middle rounds, the underdog had kept it close, creating difficulties for Buatsi, who utilized a tight guard to barely take the cleaner periods.
The crowd's patience wore thin during constant clinching, with the challenger slipping to the canvas various instances during clinches.
Parker ended stronger, but many rounds were difficult to score. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Motivates Local Rivalry Battle
In the secondary fight, Arthur overcame Rea by divided decision in a outstanding all-Manchester battle to win the continental belt.
The stadium became quiet for a deeply poignant tribute ahead of the contest as a bell ceremony rang in honor of local hero Hatton, who died aged 46 in last month.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime ended, the silence transitioned to a chant of "only one Ricky Hatton".
The fighting inside the ropes equaled the sentiment. The eventual winner was sent to the canvas by a left hook directly to the jaw in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up soon, heavy swelling forming under his eye as the fighters traded powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur edged the verdict with judges scorecards of draw, narrow lead and wider gap, but both fighters were determined and courageous, everything their idol the legend represented.
