UK light heavyweight Buatsi secured a victory yet unimpressive beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a tight 10-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective clean blows in a match where the approaches never quite clicked, with frequent clinching marring the fight.
The officials judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but how do champions respond? They win," Buatsi stated in the arena as he was faced disapproval from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Various analysts believed Parker, 31, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
Buatsi Secures Victory in Uneventful Affair
His career development has stalled in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors prepared to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the better stanzas.
The crowd's patience wore thin because of frequent holding, with the challenger slipping to the canvas several times during grappling.
Parker closed well, but many rounds were difficult to score. However, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Contest
In the secondary fight, Arthur defeated Rea by divided decision in a excellent all-Manchester contest to win the European light-heavyweight championship.
The stadium grew hushed for a deeply poignant homage before the fight as 10 bells rang in memory of the city's native Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the concluding sound faded, the silence transitioned to a roar of "there's just one Hatton".
The fighting during the bout reflected the emotion. Arthur was sent to the canvas by a significant blow squarely on the face in the second round, rising shakily to his footing on the referee's count.
But the red-colored equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up soon, heavy swelling forming under his eye as the fighters landed hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
The victor edged the verdict with judges scorecards of 114-114, 115-113 and wider gap, but each competitor were determined and brave, everything their idol the legend represented.
