UK light-heavyweight Buatsi secured a victory yet disappointing start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round fight at the boxing arena.
But Parker had his moments, landing some telling single shots in a match where the approaches failed to mesh, with excessive holding spoiling the contest.
Two judges scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest display but how do champions respond? They win," Buatsi stated in the arena as he was faced disapproval from the crowd.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Various analysts thought the challenger, 31, should have been awarded the win but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
Buatsi Earns Victory in Uneventful Affair
Buatsi's progress has plateaued in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter prepared to be aggressive in a tentative initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who operated with a protective stance to narrowly win the cleaner periods.
The spectators' acceptance decreased amid constant clinching, with Parker falling to the floor multiple occasions during grappling.
The underdog closed well, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor advances, but with the presence of hard hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester contest to win the European light heavyweight championship.
The stadium fell silent for a deeply poignant homage before the fight as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who died 46 years old in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound ended, the quiet transitioned to a roar of "there's just one Hatton".
The fighting within the ring equaled the emotion. The eventual winner was knocked down by a left hook directly to the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the red-colored attire of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was marked up soon, significant bruising appearing under his face as the pair exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over the full distance.
The victor edged the result with scoring tallies of draw, 115-113 and 115-112, but both fighters were determined and full of heart, all the qualities their hero Hatton represented.
