UK light-heavyweight Buatsi secured a victory yet disappointing debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a match where the approaches never quite clicked, with frequent clinching spoiling the contest.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They win," Buatsi stated in the ring as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Several boxing experts believed Parker, 31, merited the victory but the contender experienced the second setback of his 28-fight professional journey.
The Champion Claims Victory in Uneventful Affair
Buatsi's progress has stalled in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under recent support.
He began cautiously, with neither fighter willing to be aggressive in a tentative early stages.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to edge the better periods.
The audience's tolerance diminished because of frequent holding, with Parker slipping to the canvas various instances during close exchanges.
Parker closed well, but many rounds were difficult to score. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester battle to win the continental title.
The arena fell silent for a profoundly moving homage prior to the bout as 10 bells sounded in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the concluding sound ended, the silence transitioned to a chant of "there's only one Ricky Hatton".
The competition within the ring reflected the sentiment. Arthur was knocked down by a left hook directly to the jaw in the second round, rising shakily to his footing on the eight count.
But the traditional red shorts of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was marked up early, noticeable damage appearing under his face as the duo traded hard punches.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the action rarely dropped over twelve rounds.
Arthur barely secured the result with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were determined and brave, exactly what their inspiration the legend embodied.
