UK light heavyweight Buatsi secured a victory yet disappointing start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a split decision in a close ten-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a contest where the styles never quite clicked, with frequent clinching spoiling the fight.
Two judges had it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but what do champions do? They prevail," the victor commented in the arena as he was met by disapproval from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a twentieth professional win.
Several boxing experts thought Parker, 31, merited the win but the British boxer experienced the second loss of his 28-fight career.
Buatsi Claims Triumph in Uneventful Encounter
Buatsi's progress has slowed in recent times through contract problems and long spells of lack of fights.
Once tipped as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under recent support.
He started carefully, with the two boxers willing to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who worked behind a solid defense to barely take the more effective rounds.
The audience's tolerance wore thin because of constant clinching, with the challenger falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. Although, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Memorial Fuels All-Manchester Fight
In the supporting bout, Arthur overcame Bradley Rea by close verdict in a excellent all-Manchester contest to secure the continental title.
The venue became quiet for a deeply poignant homage ahead of the contest as 10 bells tolled in honor of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the final chime faded, the quiet gave way to a roar of "there's just one Ricky Hatton".
The competition within the ring matched the feeling. The eventual winner was sent to the canvas by a left hook flush on the face in the initial stage, getting up unsteadily to his position on the eight count.
But the red-colored shorts of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was marked up quickly, significant bruising appearing under his face as the fighters exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur edged the verdict with official scores of draw, close margin and clearer margin, but both fighters were relentless and courageous, all the qualities their inspiration the legend represented.
