British light heavyweight Buatsi secured a victory yet disappointing debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a tight 10-round fight at the boxing arena.
But Parker had his moments, delivering effective individual punches in a fight where the approaches never quite clicked, with frequent clinching marring the action.
Two judges judged it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi stated in the ringside as he was faced boos from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to earn a 20th pro success.
Several boxing experts felt the challenger, thirty-one, should have been awarded the victory but the contender experienced the second defeat of his 28-fight career.
The Champion Secures Victory in Uneventful Encounter
His career development has slowed in the past few years through management issues and considerable time of lack of fights.
Once tipped as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter willing to take risks in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was making just his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who utilized a solid defense to edge the more effective periods.
The audience's tolerance wore thin during constant clinching, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog finished the stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the supporting bout, Arthur overcame Rea by close verdict in a outstanding all-Manchester contest to secure the European light heavyweight championship.
The venue fell silent for a deeply poignant memorial before the fight as a bell ceremony tolled in honor of the city's native Hatton, who passed away aged 46 in September.
It was the first major televised boxing event in the location since Hatton's death. As the concluding sound faded, the quiet gave way to a chant of "only one Ricky Hatton".
The fighting inside the ropes equaled the sentiment. The eventual winner was floored by a significant blow squarely on the chin in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red attire of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged early, heavy swelling developing under his orbital area as the duo exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the pace rarely dropped over the full distance.
Arthur edged the decision with scoring tallies of even score, narrow lead and clearer margin, but the two athletes were determined and courageous, all the qualities their idol the legend represented.
