UK light heavyweight Buatsi made a winning yet underwhelming beginning to his career under new promoters with a points victory 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 Manchester venue.
But Parker landed significant shots, landing some telling single shots in a contest where the approaches failed to mesh, with excessive holding disrupting the contest.
The scoring judges scored it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best display but how do champions respond? They succeed," Buatsi said in the arena as he was greeted with boos from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts felt the challenger, 31, merited the victory but the Derbyshire fighter endured the second defeat of his 28-match career.
Buatsi Secures Win in Uneventful Match
The fighter's advancement has stalled in recent times through contract problems and considerable time of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - possibly in what feels like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to take risks in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to narrowly win the cleaner stanzas.
The audience's tolerance diminished amid regular grabbing, with Parker going down multiple occasions during clinches.
Parker closed well, but many rounds were difficult to score. However, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor advances, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Hatton Tribute Fuels All-Manchester Battle
In the supporting bout, Arthur beat Bradley Rea by close verdict in a excellent local rivalry fight to capture the European light-heavyweight title.
The venue became quiet for a extremely emotional memorial ahead of the contest as a bell ceremony sounded in honor of Manchester's own Hatton, who passed away 46 years old in recently.
It was the first major televised boxing event in the city since the passing of Hatton. As the final chime ended, the quiet transitioned to a roar of "only one Ricky Hatton".
The action during the bout equaled the emotion. The eventual winner was floored by a significant blow flush on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was damaged soon, significant bruising forming under his face as the duo landed hard punches.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with effective uppercuts as the action hardly diminished over twelve rounds.
Arthur narrowly won the result with official scores of 114-114, close margin and clearer margin, but both fighters were relentless and full of heart, everything their inspiration Hatton embodied.
