British light-heavyweight Buatsi made a winning yet underwhelming beginning to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close ten-round contest at the boxing arena.
But the challenger had his moments, landing some telling clean blows in a fight where the styles failed to mesh, with frequent clinching spoiling the contest.
Two judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest performance but what defines champions? They succeed," Buatsi said in the ringside as he was met by disapproval from the audience.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Various analysts believed Parker, 31, deserved the win but the British boxer endured the second defeat of his 28-match professional journey.
Buatsi Claims Win in Dull Affair
The fighter's advancement has slowed in recent times through contract problems and extended periods of lack of fights.
Once tipped as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under recent support.
He started carefully, with both competitors ready to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second bout at light heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who utilized a protective stance to narrowly win the better periods.
The audience's tolerance decreased because of frequent holding, with the challenger falling to the floor various instances during clinches.
Parker closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Contest
In the supporting bout, Arthur overcame Bradley Rea by close verdict in a excellent local rivalry battle to capture the continental title.
The arena grew hushed for a profoundly moving memorial ahead of the contest as ten bells rang in memory of Manchester's own Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast fight night in the location since Hatton's death. As the concluding sound ended, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The competition during the bout reflected the emotion. Arthur was floored by a left hook flush on the jaw in the early round, rising shakily to his feet on the eight count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was damaged soon, significant bruising developing under his orbital area as the fighters traded hard punches.
The challenger, 27, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor barely secured the result with official scores of draw, 115-113 and clearer margin, but each competitor were determined and full of heart, all the qualities their inspiration Hatton embodied.
