British light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a fight where the styles didn't complement each other, with excessive holding spoiling the action.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my finest showing but what do champions do? They prevail," Buatsi said in the arena as he was met by boos from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Various analysts believed Parker, 31, should have been awarded the decision but the British boxer suffered the second setback of his 28-bout career.
Buatsi Earns Triumph in Slow Encounter
The fighter's advancement has stalled in the past few years through contract problems and long spells of lack of fights.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter prepared to overcommit in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to narrowly win the better periods.
The audience's tolerance decreased during constant clinching, with Parker going down several times during grappling.
Parker ended stronger, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a excellent Manchester vs Manchester fight to capture the European light heavyweight title.
The arena grew hushed for a extremely emotional memorial prior to the bout as ten bells rang in honor of local hero Hatton, who passed away at age 46 in last month.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound disappeared, the quiet became a roar of "there's only one Ricky Hatton".
The action during the bout reflected the emotion. The eventual winner was knocked down by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored attire of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was injured soon, noticeable damage forming under his eye as the duo exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over twelve rounds.
Arthur narrowly won the verdict with scoring tallies of even score, 115-113 and clearer margin, but both fighters were persistent and full of heart, everything their idol the legend stood for.
