UK light-heavyweight Buatsi earned a win yet unimpressive start to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive 10-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful individual punches in a contest where the approaches never quite clicked, with frequent clinching spoiling the action.
The officials had it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but what do champions do? They succeed," the victor said in the ring as he was met by negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts felt Parker, thirty-one, merited the victory but the Derbyshire fighter experienced the second loss of his 28-fight professional journey.
The Champion Earns Victory in Slow Match
The fighter's advancement has stalled in recent years through management issues and long spells of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers ready to take risks in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a tight guard to edge the better stanzas.
The audience's tolerance wore thin because of constant clinching, with the challenger going down several times during clinches.
The underdog ended stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Arthur beat Rea by divided decision in a excellent local rivalry contest to secure the European light heavyweight title.
The venue grew hushed for a deeply poignant memorial before the fight as ten bells rang in honor of the city's native Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised boxing event in the location since Hatton's death. As the concluding sound faded, the silence gave way to a chant of "there's just one Ricky Hatton".
The fighting within the ring reflected the sentiment. Arthur was floored by a significant blow squarely on the jaw in the early round, standing uncertainly to his position on the eight count.
But the Manchester United-red shorts of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was injured soon, significant bruising developing under his orbital area as the fighters landed hard punches.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the pace seldom decreased over 12 rounds.
The victor edged the verdict with official scores of draw, narrow lead and 115-112, but the two athletes were persistent and full of heart, everything their hero Hatton stood for.
