UK light-heavyweight Buatsi made a winning yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a competitive 10-round contest at the Manchester venue.
But the challenger had his moments, connecting with powerful individual punches in a match where the approaches failed to mesh, with frequent clinching disrupting the contest.
The officials had it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what defines champions? They win," the victor commented in the ringside as he was met by boos from the audience.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Various analysts thought Parker, 31, should have been awarded the victory but the British boxer experienced the second defeat of his 28-fight professional journey.
The Champion Earns Win in Dull Affair
The fighter's advancement has plateaued in recent years through management issues and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to realize his potential - possibly in what seems like a final roll of the dice - under recent support.
He opened tentatively, with both competitors prepared to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was participating in his second bout at light heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who worked behind a protective stance to narrowly win the cleaner rounds.
The crowd's patience decreased because of regular grabbing, with Parker going down several times during close exchanges.
The underdog closed well, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor moves on, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the secondary fight, Arthur beat Rea by close verdict in a excellent Manchester vs Manchester battle to secure the continental title.
The arena became quiet for a deeply poignant tribute before the fight as ten bells tolled in remembrance of local hero Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound disappeared, the silence gave way to a roar of "there's just one Ricky Hatton".
The competition within the ring matched the feeling. The eventual winner was sent to the canvas by a significant blow flush on the chin in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was damaged early, significant bruising developing under his orbital area as the duo landed powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over twelve rounds.
Arthur edged the decision with official scores of draw, close margin and clearer margin, but each competitor were relentless and brave, everything their inspiration Hatton represented.
