British light heavyweight Buatsi earned a win yet disappointing debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger found success, landing some telling single shots in a fight where the styles never quite clicked, with constant grabbing marring the fight.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what do champions do? They prevail," Buatsi said in the arena as he was faced negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, merited the win but the contender suffered the second loss of his 28-bout career.
The Champion Earns Victory in Slow Match
The fighter's advancement has plateaued in recent years through contract problems and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to achieve expectations - possibly in what seems like a final roll of the dice - under fresh management.
He started carefully, with both competitors prepared to take risks in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who operated with a solid defense to barely take the better rounds.
The spectators' acceptance decreased because of frequent holding, with the challenger going down multiple occasions during grappling.
Parker finished the stronger, but multiple periods were difficult to score. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Arthur overcame Rea by divided decision in a outstanding all-Manchester battle to win the European light heavyweight championship.
The stadium grew hushed for a profoundly moving homage ahead of the contest as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who died 46 years old in recently.
It was the first major televised boxing event in the urban center since Hatton's death. As the last bell faded, the silence gave way to a roar of "only one Hatton".
The fighting within the ring matched the feeling. Arthur was knocked down by a left hook squarely on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was marked up early, heavy swelling forming under his face as the duo traded powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
The victor edged the result with judges scorecards of 114-114, 115-113 and 115-112, but both fighters were determined and courageous, everything their inspiration the legend embodied.
