UK light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a tight 10-round fight at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a match where the techniques never quite clicked, with excessive holding spoiling the action.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my greatest showing but what do champions do? They succeed," the victor stated in the arena as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Various analysts felt Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter endured the second defeat of his 28-match professional journey.
The Champion Earns Win in Dull Encounter
Buatsi's progress has stalled in the past few years through management issues and long spells of inactivity.
Once tipped as the future of British boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with both competitors ready to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second fight at light-heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who worked behind a protective stance to barely take the better stanzas.
The audience's tolerance wore thin amid constant clinching, with Parker slipping to the canvas various instances during grappling.
The underdog ended stronger, but many rounds were hard to judge. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor advances, but with the likes of heavy hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the co-main event, Arthur beat Rea by split-decision in a excellent local rivalry fight to win the European light heavyweight title.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as 10 bells rang in memory of the city's native Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the final chime faded, the silence gave way to a chant of "there's only one Ricky Hatton".
The fighting within the ring equaled the emotion. The eventual winner was sent to the canvas by a left hook directly to the chin in the second round, getting up unsteadily to his feet on the count of eight.
But the red-colored attire of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was marked up early, noticeable damage developing under his face as the duo exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the decision with scoring tallies of even score, close margin and 115-112, but the two athletes were determined and courageous, exactly what their inspiration Hatton represented.
