British light heavyweight Buatsi made a winning yet underwhelming start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful individual punches in a match where the techniques failed to mesh, with excessive holding marring the contest.
The officials had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," the victor stated in the arena as he was greeted with boos from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts felt the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
The Champion Secures Victory in Dull Affair
The fighter's advancement has slowed in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the future of British boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a last chance - under recent support.
He started carefully, with the two boxers willing to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a solid defense to narrowly win the better rounds.
The crowd's patience decreased amid regular grabbing, with Parker slipping to the canvas several times during clinches.
Parker closed well, but multiple periods were hard to judge. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Fight
In the co-main event, Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester fight to win the European light-heavyweight belt.
The stadium grew hushed for a profoundly moving tribute before the fight as 10 bells rang in remembrance of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised match card in the city since the passing of Hatton. As the concluding sound ended, the quiet gave way to a chant of "only one Hatton".
The competition inside the ropes reflected the emotion. Arthur was knocked down by a powerful punch squarely on the chin in the initial stage, standing uncertainly to his position on the count of eight.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was injured quickly, noticeable damage developing under his eye as the pair exchanged punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over the full distance.
Arthur barely secured the result with scoring tallies of even score, 115-113 and 115-112, but both fighters were determined and brave, all the qualities their idol the legend stood for.
