UK light-heavyweight Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a competitive ten-round fight at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the styles never quite clicked, with frequent clinching spoiling the fight.
Two judges scored it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi commented in the ringside as he was faced negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts felt the challenger, thirty-one, merited the decision but the British boxer experienced the second defeat of his 28-bout professional journey.
The Champion Claims Win in Dull Affair
The fighter's advancement has plateaued in recent times through contract problems and extended periods of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a last chance - under new promotional backing.
He started carefully, with both competitors ready to be aggressive in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who worked behind a tight guard to narrowly win the better stanzas.
The audience's tolerance decreased because of regular grabbing, with the challenger going down multiple occasions during grappling.
Parker closed well, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Homage Fuels Local Rivalry Contest
In the supporting bout, Arthur defeated Rea by close verdict in a outstanding local rivalry fight to secure the continental championship.
The venue became quiet for a profoundly moving tribute before the fight as a bell ceremony tolled in remembrance of the city's native Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the city since Hatton's passing. As the concluding sound faded, the quiet transitioned to a chant of "there's just one Hatton".
The fighting inside the ropes reflected the sentiment. The eventual winner was floored by a powerful punch directly to the chin in the early round, getting up unsteadily to his position on the count of eight.
But the red-colored equipment of Arthur soon came forward again. Across the ring, the opponent - in opposing blue - was injured early, significant bruising appearing under his eye as the fighters exchanged punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over twelve rounds.
The victor edged the verdict with official scores of 114-114, close margin and wider gap, but both fighters were persistent and courageous, exactly what their inspiration Hatton represented.
