British light-heavyweight Buatsi earned a win yet underwhelming debut to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a tight 10-round bout at the boxing arena.
But the challenger had his moments, connecting with powerful single shots in a match where the styles didn't complement each other, with constant grabbing disrupting the fight.
The officials judged it 96-94 for to the winner, with the third a even score.
"It wasn't my best performance but what defines champions? They succeed," the victor stated in the arena as he was greeted with disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers believed Parker, 31, should have been awarded the win but the contender experienced the second defeat of his 28-bout professional journey.
The Champion Secures Win in Dull Affair
Buatsi's progress has stalled in recent years through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to realize his potential - maybe in what appears to be a last chance - under fresh management.
He began cautiously, with the two boxers willing to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who operated with a tight guard to barely take the more effective periods.
The spectators' acceptance diminished because of constant clinching, with Parker falling to the floor various instances during close exchanges.
The underdog closed well, but several stanzas were hard to judge. Although, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Homage Fuels Manchester Derby Battle
In the co-main event, Arthur beat Rea by divided decision in a excellent all-Manchester fight to secure the European light heavyweight championship.
The venue grew hushed for a profoundly moving homage before the fight as a bell ceremony sounded in honor of the city's native Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime disappeared, the silence gave way to a shout of "there's just one Ricky Hatton".
The fighting within the ring reflected the emotion. Arthur was knocked down by a powerful punch directly to the face in the initial stage, standing uncertainly to his position on the count of eight.
But the red-colored equipment of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was injured quickly, significant bruising developing under his face as the fighters traded hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over twelve rounds.
Arthur narrowly won the decision with scoring tallies of 114-114, 115-113 and clearer margin, but both fighters were determined and courageous, everything their hero the legend stood for.
