British light heavyweight Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a tight 10-round contest at the Manchester venue.
But the challenger found success, delivering effective single shots in a contest where the approaches didn't complement each other, with frequent clinching spoiling the action.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They succeed," the victor said in the ring as he was greeted with boos from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts felt Parker, 31, merited the win but the Derbyshire fighter experienced the second defeat of his 28-fight career.
Buatsi Secures Triumph in Dull Encounter
Buatsi's progress has slowed in recent years through management issues and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers prepared to be aggressive in a tentative opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who worked behind a tight guard to narrowly win the more effective stanzas.
The crowd's patience decreased during constant clinching, with the challenger going down several times during clinches.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Fuels All-Manchester Fight
In the secondary fight, Arthur beat Rea by split-decision in a terrific local rivalry contest to secure the continental title.
The stadium became quiet for a profoundly moving memorial ahead of the contest as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the last bell faded, the silence became a chant of "there's just one Ricky Hatton".
The fighting inside the ropes reflected the sentiment. The eventual winner was knocked down by a left hook directly to the face in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was injured soon, noticeable damage developing under his face as the fighters traded powerful shots.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over twelve rounds.
Arthur narrowly won the result with scoring tallies of even score, narrow lead and clearer margin, but each competitor were persistent and full of heart, all the qualities their hero the legend represented.
