UK light heavyweight Buatsi made a winning yet disappointing start to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a match where the styles didn't complement each other, with excessive holding disrupting the action.
The officials had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest display but what do champions do? They win," the victor said in the ring as he was met by disapproval from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers thought Parker, thirty-one, should have been awarded the win but the contender experienced the second defeat of his 28-fight career.
The Champion Claims Victory in Slow Affair
Buatsi's progress has stalled in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a calculated opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who utilized a tight guard to edge the better stanzas.
The spectators' acceptance diminished during constant clinching, with the challenger going down several times during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. Although, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Arthur beat Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to win the European light-heavyweight title.
The stadium became quiet for a profoundly moving tribute ahead of the contest as ten bells tolled in honor of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime faded, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The fighting during the bout matched the feeling. The eventual winner was knocked down by a powerful punch flush on the chin in the second round, rising shakily to his position on the count of eight.
But the traditional red attire of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was marked up early, noticeable damage forming under his face as the pair landed hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the verdict with official scores of even score, narrow lead and clearer margin, but both fighters were relentless and brave, exactly what their idol Hatton embodied.
