UK light heavyweight Buatsi earned a win yet disappointing debut to life under new promoters 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 the challenger found success, connecting with powerful clean blows in a fight where the styles never quite clicked, with frequent clinching disrupting the action.
Two judges judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest performance but what defines champions? They succeed," the victor stated in the arena as he was greeted with disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Various analysts thought the challenger, 31, should have been awarded the win but the British boxer suffered the second setback of his 28-fight professional journey.
The fighter's advancement has stalled in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to achieve expectations - perhaps in what seems like a final roll of the dice - under recent support.
He began cautiously, with both competitors prepared to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who utilized a tight guard to edge the cleaner periods.
The crowd's patience decreased because of constant clinching, with the challenger going down multiple occasions during close exchanges.
The underdog closed well, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
In the secondary fight, Arthur beat Rea by divided decision in a excellent Manchester vs Manchester contest to capture the European light heavyweight belt.
The venue fell silent for a profoundly moving memorial before the fight as ten bells tolled in remembrance of the city's native Hatton, who passed away 46 years old in September.
It was the premier televised match card in the location since the passing of Hatton. As the concluding sound faded, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The fighting inside the ropes equaled the feeling. The eventual winner was floored by a powerful punch directly to the chin in the early round, rising shakily to his footing on the eight count.
But the traditional red shorts of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was injured quickly, noticeable damage forming under his orbital area as the pair exchanged punishing blows.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the pace seldom decreased over 12 rounds.
Arthur edged the verdict with judges scorecards of 114-114, 115-113 and wider gap, but each competitor were determined and brave, all the qualities their inspiration Hatton embodied.
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