British light heavyweight Buatsi secured a victory yet underwhelming start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive 10-round contest at the Manchester venue.
But Parker had his moments, delivering effective single shots in a match where the approaches never quite clicked, with frequent clinching disrupting the fight.
Two judges judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but how do champions respond? They win," the victor said in the ring as he was greeted with disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought Parker, 31, should have been awarded the win but the British boxer experienced the second loss of his 28-fight professional journey.
The Champion Secures Victory in Dull Encounter
Buatsi's progress has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He began cautiously, with both competitors prepared to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a protective stance to edge the cleaner stanzas.
The crowd's patience decreased because of constant clinching, with the challenger falling to the floor several times during grappling.
Parker ended stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific Manchester vs Manchester contest to secure the European light-heavyweight title.
The venue became quiet for a profoundly moving memorial prior to the bout as a bell ceremony tolled in memory of local hero Ricky Hatton, who died 46 years old in last month.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime faded, the stillness transitioned to a roar of "only one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was knocked down by a powerful punch directly to the chin in the early round, rising shakily to his feet on the eight count.
But the red-colored shorts of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was damaged early, heavy swelling forming under his face as the pair landed hard punches.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the verdict with official scores of even score, narrow lead and wider gap, but both fighters were relentless and full of heart, all the qualities their idol the legend stood for.
