UK light-heavyweight Buatsi secured a victory yet underwhelming debut to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a tight ten-round fight at the Co-op Live Arena.
But Parker found success, delivering effective single shots in a fight where the styles never quite clicked, with frequent clinching disrupting the contest.
The officials judged it 96-94 for to the winner, with the third a even score.
"It wasn't my best display but what defines champions? They win," the victor commented in the arena as he was met by disapproval from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Some ringside observers thought Parker, 31, merited the win but the contender suffered the second defeat of his 28-bout professional journey.
Buatsi Earns Win in Slow Affair
The fighter's advancement has slowed in recent years through management issues and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers prepared to take risks in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who operated with a solid defense to edge the more effective periods.
The spectators' acceptance decreased during frequent holding, with the challenger falling to the floor various instances during clinches.
Parker closed well, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor advances, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Motivates Manchester Derby Contest
In the co-main event, Lyndon Arthur overcame Rea by divided decision in a outstanding all-Manchester battle to capture the European light-heavyweight title.
The stadium fell silent for a profoundly moving memorial ahead of the contest as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the last bell ended, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The competition inside the ropes equaled the sentiment. The eventual winner was floored by a left hook flush on the face in the second round, getting up unsteadily to his position on the eight count.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was injured soon, heavy swelling appearing under his face as the duo traded powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace seldom decreased over 12 rounds.
The victor barely secured the result with judges scorecards of 114-114, narrow lead and 115-112, but the two athletes were determined and brave, everything their inspiration Hatton stood for.
