British light-heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a close ten-round bout at the Manchester venue.
But the challenger had his moments, delivering effective single shots in a contest where the techniques didn't complement each other, with frequent clinching spoiling the fight.
Two judges had it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my best performance but what do champions do? They prevail," the victor stated in the arena as he was faced disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts thought the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Claims Victory in Dull Affair
Buatsi's progress has stalled in recent years through contract problems and considerable time of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to achieve expectations - possibly in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors prepared to overcommit in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who worked behind a tight guard to barely take the cleaner stanzas.
The audience's tolerance decreased because of constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
The underdog closed well, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Fight
In the secondary fight, Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester contest to secure the European light heavyweight championship.
The arena grew hushed for a deeply poignant homage before the fight as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the location since Hatton's death. As the concluding sound ended, the stillness became a chant of "only one Ricky Hatton".
The competition during the bout reflected the sentiment. The eventual winner was floored by a significant blow directly to the chin in the early round, standing uncertainly to his position on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up soon, heavy swelling forming under his eye as the pair traded punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the action rarely dropped over the full distance.
The victor barely secured the verdict with official scores of even score, narrow lead and clearer margin, but both fighters were relentless and courageous, exactly what their idol the legend represented.
