British light-heavyweight Buatsi secured a victory yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the approaches never quite clicked, with frequent clinching marring the action.
The officials scored it 96-94 to Buatsi, with the third a even score.
"It wasn't my best performance but what defines champions? They succeed," the victor commented in the arena as he was faced negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Several boxing experts felt the challenger, 31, deserved the win but the Derbyshire fighter endured the second setback of his 28-fight professional journey.
Buatsi Secures Win in Slow Match
His career development has slowed in recent times through contract problems and long spells of lack of fights.
Once tipped as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter willing to be aggressive in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to barely take the cleaner stanzas.
The crowd's patience diminished during regular grabbing, with Parker slipping to the canvas various instances during grappling.
The underdog closed well, but multiple periods were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Hatton Memorial Motivates Local Rivalry Contest
In the co-main event, Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester battle to capture the continental title.
The venue became quiet for a profoundly moving memorial before the fight as 10 bells rang in memory of the city's native Hatton, who died 46 years old in last month.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the final chime disappeared, the stillness became a shout of "only one Ricky Hatton".
The competition within the ring reflected the feeling. The eventual winner was floored by a left hook directly to the chin in the early round, rising shakily to his footing on the count of eight.
But the traditional red equipment of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was injured early, noticeable damage developing under his eye as the pair landed powerful shots.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur narrowly won the decision with scoring tallies of draw, close margin and clearer margin, but the two athletes were determined and full of heart, all the qualities their hero Hatton represented.
