British light-heavyweight Buatsi earned a win yet disappointing beginning to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a contest where the approaches never quite clicked, with excessive holding marring the fight.
The scoring judges had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my finest display but what defines champions? They prevail," Buatsi stated in the ring as he was met by negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts believed Parker, 31, should have been awarded the decision but the contender suffered the second loss of his 28-fight career.
The Champion Earns Victory in Dull Encounter
His career development has plateaued in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with both competitors willing to be aggressive in a tentative initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was participating in his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who utilized a protective stance to narrowly win the better periods.
The spectators' acceptance decreased during regular grabbing, with Parker going down multiple occasions during clinches.
The underdog closed well, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a outstanding Manchester vs Manchester fight to win the European light-heavyweight belt.
The stadium became quiet for a deeply poignant memorial ahead of the contest as a bell ceremony rang in memory of Manchester's own Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the concluding sound ended, the stillness gave way to a chant of "only one Hatton".
The action within the ring equaled the sentiment. The eventual winner was knocked down by a left hook directly to the jaw in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red equipment of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was injured quickly, heavy swelling forming under his eye as the fighters landed punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur edged the decision with judges scorecards of draw, 115-113 and wider gap, but the two athletes were relentless and brave, all the qualities their inspiration Hatton embodied.
