British light-heavyweight Buatsi made a winning yet disappointing start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close 10-round bout at the boxing arena.
But the challenger found success, delivering effective individual punches in a match where the techniques failed to mesh, with excessive holding marring the contest.
The officials had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest display but how do champions respond? They prevail," the victor stated in the ring as he was faced negative reactions from the crowd.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the win but the Derbyshire fighter experienced the second defeat of his 28-bout professional journey.
Buatsi Earns Triumph in Uneventful Encounter
Buatsi's progress has plateaued in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to realize his potential - maybe in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers prepared to be aggressive in a cagey initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who worked behind a solid defense to edge the cleaner rounds.
The audience's tolerance diminished because of regular grabbing, with Parker going down various instances during clinches.
The underdog ended stronger, but multiple periods were difficult to score. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a outstanding local rivalry contest to capture the European light heavyweight title.
The arena grew hushed for a extremely emotional homage ahead of the contest as 10 bells rang in honor of local hero Hatton, who passed away at age 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the concluding sound ended, the silence became a chant of "there's only one Hatton".
The competition within the ring reflected the emotion. Arthur was knocked down by a left hook directly to the chin in the early round, getting up unsteadily to his footing on the referee's count.
But the traditional red equipment of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was marked up quickly, significant bruising forming under his face as the fighters traded powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor edged the verdict with judges scorecards of 114-114, narrow lead and 115-112, but the two athletes were relentless and courageous, all the qualities their hero Hatton embodied.
