UK light heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a split decision in a tight ten-round fight at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a fight where the approaches didn't complement each other, with frequent clinching spoiling the fight.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They win," Buatsi said in the arena as he was met by disapproval from the crowd.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, deserved the decision but the contender experienced the second defeat of his 28-match career.
The Champion Secures Victory in Slow Affair
The fighter's advancement has slowed in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers prepared to take risks in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who utilized a solid defense to narrowly win the cleaner rounds.
The spectators' acceptance decreased amid constant clinching, with Parker going down multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Arthur defeated Rea by divided decision in a outstanding all-Manchester fight to capture the continental belt.
The venue became quiet for a profoundly moving tribute ahead of the contest as ten bells rang in honor of Manchester's own Hatton, who succumbed at age 46 in September.
It was the first major televised boxing event in the city since Hatton's death. As the last bell ended, the stillness gave way to a shout of "only one Ricky Hatton".
The fighting within the ring matched the feeling. Arthur was sent to the canvas by a powerful punch squarely on the chin in the second round, rising shakily to his footing on the referee's count.
But the traditional red shorts of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was injured early, noticeable damage forming under his face as the fighters landed hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor edged the verdict with official scores of even score, 115-113 and wider gap, but each competitor were determined and courageous, exactly what their hero Hatton represented.
