British light heavyweight Buatsi earned a win yet disappointing start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a split decision in a competitive 10-round contest at the Manchester venue.
But Parker found success, delivering effective single shots in a match where the styles didn't complement each other, with constant grabbing marring the fight.
Two judges scored it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but what do champions do? They succeed," the victor said in the ring as he was met by disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th career victory.
Various analysts thought the challenger, 31, deserved the victory but the Derbyshire fighter suffered the second defeat of his 28-fight career.
The Champion Claims Victory in Uneventful Encounter
The fighter's advancement has stalled in recent times through contract problems and considerable time of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a last chance - under fresh management.
He began cautiously, with neither fighter willing to take risks in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second appearance at light heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to narrowly win the better rounds.
The crowd's patience diminished because of frequent holding, with Parker slipping to the canvas various instances during grappling.
Parker closed well, but many rounds were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester battle to secure the European light-heavyweight title.
The stadium became quiet for a extremely emotional homage prior to the bout as 10 bells sounded in memory of local hero Ricky Hatton, who passed away 46 years old in September.
It was the first major televised match card in the location since Hatton's death. As the last bell faded, the quiet became a roar of "there's just one Hatton".
The competition within the ring equaled the emotion. Arthur was floored by a significant blow squarely on the jaw in the second round, getting up unsteadily to his feet on the referee's count.
But the red-colored shorts of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was marked up soon, heavy swelling developing under his face as the pair exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the pace seldom decreased over twelve rounds.
Arthur edged the verdict with official scores of draw, close margin and clearer margin, but each competitor were relentless and full of heart, all the qualities their idol Hatton embodied.
