British light-heavyweight Buatsi secured a victory yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round fight at the boxing arena.
But the challenger found success, connecting with powerful clean blows in a fight where the styles didn't complement each other, with constant grabbing disrupting the contest.
The officials judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi stated in the ring as he was greeted with boos from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, deserved the win but the contender endured the second setback of his 28-bout professional journey.
Buatsi Claims Triumph in Dull Match
His career development has stalled in recent times through management issues and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a tentative initial 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 appearance at light heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who worked behind a solid defense to barely take the cleaner stanzas.
The audience's tolerance diminished because of constant clinching, with Parker going down multiple occasions during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. Although, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Tribute Motivates Manchester Derby Fight
In the supporting bout, Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to win the European light-heavyweight belt.
The stadium fell silent for a profoundly moving homage before the fight as 10 bells sounded in memory of the city's native Ricky Hatton, who died aged 46 in last month.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound disappeared, the silence became a chant of "there's just one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was knocked down by a left hook directly to the jaw in the second round, rising shakily to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, the opponent - in competitive blue - was marked up quickly, significant bruising developing under his face as the duo traded punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor narrowly won the verdict with judges scorecards of even score, narrow lead and 115-112, but the two athletes were determined and full of heart, exactly what their idol the legend represented.
