British light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close 10-round bout at the boxing arena.
But Parker had his moments, delivering effective clean blows in a match where the styles never quite clicked, with excessive holding marring the action.
Two judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest showing but what do champions do? They win," the victor said in the arena as he was faced negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Several boxing experts believed the challenger, 31, merited the win but the British boxer suffered the second defeat of his 28-bout professional journey.
Buatsi Secures Triumph in Dull Affair
His career development has stalled in recent times through contract problems and long spells of lack of fights.
Once tipped as the future of the British fight scene, he now finds himself trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under fresh management.
He began cautiously, with both competitors willing to overcommit in a calculated early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who operated with a solid defense to barely take the cleaner rounds.
The crowd's patience wore thin amid frequent holding, with Parker going down multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Motivates All-Manchester Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding all-Manchester contest to secure the continental title.
The arena became quiet for a profoundly moving tribute before the fight as ten bells tolled in honor of the city's native Ricky Hatton, who died aged 46 in September.
It was the first major televised fight night in the urban center since the passing of Hatton. As the concluding sound ended, the silence became a shout of "there's only one Ricky Hatton".
The action within the ring reflected the emotion. Arthur was sent to the canvas by a powerful punch flush on the chin in the second round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. Across the ring, the opponent - in competitive blue - was injured early, significant bruising forming under his eye as the duo landed hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur edged the result with scoring tallies of 114-114, 115-113 and clearer margin, but the two athletes were relentless and brave, exactly what their inspiration the legend represented.
