British light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a close ten-round contest at the Manchester venue.
But Parker had his moments, delivering effective individual punches in a fight where the techniques failed to mesh, with frequent clinching spoiling the fight.
Two judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They win," Buatsi said in the ring as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to secure a 20th pro success.
Some ringside observers felt Parker, 31, merited the win but the contender endured the second setback of his 28-fight career.
The Champion Claims Victory in Dull Encounter
The fighter's advancement has plateaued in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what appears to be a last chance - under recent support.
He started carefully, with both competitors willing to take risks in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was competing in only 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 solid defense to edge the better periods.
The spectators' acceptance diminished amid constant clinching, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Tribute Motivates Manchester Derby Battle
In the secondary fight, Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester battle to capture the continental title.
The venue fell silent for a deeply poignant homage ahead of the contest as a bell ceremony sounded in honor of the city's native Ricky Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the last bell faded, the silence became a shout of "only one Ricky Hatton".
The action within the ring matched the emotion. The eventual winner was floored by a significant blow directly to the chin in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red attire of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was injured soon, heavy swelling appearing under his eye as the pair traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the action rarely dropped over the full distance.
Arthur edged the decision with scoring tallies of even score, close margin and clearer margin, but both fighters were determined and courageous, all the qualities their idol the legend embodied.
