UK light heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling clean blows in a match where the techniques never quite clicked, with excessive holding spoiling the contest.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi commented in the ring as he was faced negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Various analysts believed Parker, 31, deserved the decision but the British boxer suffered the second loss of his 28-fight professional journey.
The Champion Secures Triumph in Uneventful Match
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of lack of fights.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to be aggressive in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who utilized a protective stance to narrowly win the cleaner rounds.
The audience's tolerance decreased because of frequent holding, with Parker falling to the floor multiple occasions during clinches.
Parker ended stronger, but multiple periods were difficult to score. However, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Memorial Motivates Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a outstanding local rivalry contest to win the continental title.
The arena grew hushed for a profoundly moving memorial before the fight as 10 bells rang in remembrance of the city's native Ricky Hatton, who died 46 years old in September.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime faded, the stillness gave way to a chant of "there's only one Ricky Hatton".
The fighting inside the ropes reflected the feeling. The eventual winner was knocked down by a left hook flush on the jaw in the early round, standing uncertainly to his position on the referee's count.
But the traditional red equipment of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was marked up quickly, noticeable damage forming under his orbital area as the fighters traded powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with powerful upward punches as the action rarely dropped over the full distance.
The victor edged the decision with scoring tallies of even score, close margin and clearer margin, but each competitor were relentless and full of heart, exactly what their inspiration Hatton represented.
