UK light-heavyweight Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a tight ten-round contest at the boxing arena.
But the challenger found success, delivering effective clean blows in a match where the techniques didn't complement each other, with constant grabbing disrupting the fight.
Two judges scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They win," Buatsi commented in the arena as he was met by boos from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers believed the challenger, 31, merited the victory but the contender endured the second setback of his 28-fight career.
Buatsi Claims Victory in Uneventful Affair
Buatsi's progress has stalled in the past few years through management issues and long spells of inactivity.
Once tipped as the future of UK boxing, he now finds himself trying to realize his potential - maybe in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers prepared to overcommit in a calculated early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a protective stance to barely take the cleaner periods.
The spectators' acceptance decreased because of regular grabbing, with the challenger slipping to the canvas various instances during clinches.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Arthur defeated Rea by close verdict in a terrific all-Manchester contest to win the European light heavyweight title.
The stadium fell silent for a extremely emotional tribute before the fight as ten bells rang in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast match card in the city since Hatton's death. As the concluding sound disappeared, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The competition within the ring reflected the sentiment. The eventual winner was sent to the canvas by a left hook directly to the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was marked up early, significant bruising appearing under his face as the fighters traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur barely secured the decision with scoring tallies of 114-114, close margin and wider gap, but the two athletes were persistent and courageous, everything their inspiration the legend stood for.
