UK light-heavyweight Buatsi made a winning yet underwhelming debut to life 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 tight ten-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective individual punches in a fight where the techniques failed to mesh, with frequent clinching spoiling the contest.
The officials had it 96-94 to the winner, with the final judge a even score.
"It wasn't my best performance but how do champions respond? They succeed," Buatsi stated in the ringside as he was faced boos from the crowd.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers felt the challenger, 31, should have been awarded the win but the contender experienced the second loss of his 28-match career.
The Champion Secures Victory in Dull Affair
His career development has plateaued in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of UK boxing, he now is positioned trying to achieve expectations - possibly in what feels like a last chance - under new promotional backing.
He opened tentatively, with neither fighter prepared to overcommit in a tentative opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a tight guard to edge the cleaner rounds.
The spectators' acceptance wore thin because of regular grabbing, with the challenger going down various instances during grappling.
The underdog ended stronger, but many rounds were difficult to score. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
The victor advances, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Hatton Homage Fuels All-Manchester Contest
In the supporting bout, Arthur defeated Rea by close verdict in a excellent local rivalry fight to secure the European light-heavyweight belt.
The stadium fell silent for a deeply poignant memorial before the fight as ten bells rang in honor of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the concluding sound disappeared, the stillness became a roar of "there's just one Ricky Hatton".
The fighting within the ring reflected the feeling. The eventual winner was knocked down by a powerful punch directly to the chin in the second round, getting up unsteadily to his feet on the eight count.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was injured early, noticeable damage appearing under his face as the fighters landed punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over the full distance.
The victor edged the verdict with judges scorecards of 114-114, close margin and wider gap, but each competitor were determined and brave, everything their hero the legend stood for.
