UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close 10-round fight at the Manchester venue.
But the challenger found success, delivering effective clean blows in a fight where the techniques failed to mesh, with excessive holding disrupting the fight.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best display but what do champions do? They prevail," Buatsi said in the ringside as he was greeted with disapproval from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th pro success.
Some ringside observers thought the challenger, 31, deserved the win but the British boxer experienced the second defeat of his 28-bout professional journey.
Buatsi Earns Victory in Dull Encounter
His career development has plateaued in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a solid defense to narrowly win the better stanzas.
The crowd's patience wore thin during constant clinching, with the challenger going down various instances during grappling.
The underdog ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester contest to capture the continental title.
The venue fell silent for a profoundly moving homage before the fight as 10 bells tolled in honor of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised fight night in the city since Hatton's death. As the final chime ended, the quiet became a shout of "there's only one Ricky Hatton".
The fighting during the bout matched the emotion. The eventual winner was floored by a significant blow directly to the face in the second round, getting up unsteadily to his position on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured early, heavy swelling appearing under his orbital area as the fighters landed powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the intensity hardly diminished over the full distance.
Arthur barely secured the decision with scoring tallies of 114-114, close margin and 115-112, but the two athletes were relentless and full of heart, all the qualities their inspiration the legend represented.
