UK light-heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a competitive ten-round fight at the boxing arena.
But the challenger landed significant shots, landing some telling individual punches in a match where the styles never quite clicked, with excessive holding disrupting the contest.
The scoring judges had it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They win," the victor said in the arena as he was met by boos from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Various analysts thought Parker, thirty-one, deserved the win but the British boxer experienced the second defeat of his 28-fight professional journey.
The Champion Claims Triumph in Slow Encounter
His career development has slowed in recent times through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to realize his potential - maybe in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter prepared to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second appearance at light-heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who worked behind a protective stance to narrowly win the more effective rounds.
The audience's tolerance decreased amid frequent holding, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. Although, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the supporting bout, Arthur overcame Bradley Rea by split-decision in a terrific Manchester vs Manchester battle to win the European light-heavyweight title.
The arena grew hushed for a extremely emotional tribute ahead of the contest as ten bells sounded in memory of local hero Ricky Hatton, who passed away at age 46 in September.
It was the first major televised boxing event in the location since the passing of Hatton. As the concluding sound ended, the stillness transitioned to a chant of "there's only one Hatton".
The fighting inside the ropes equaled the emotion. The eventual winner was floored by a powerful punch squarely on the jaw in the early round, standing uncertainly to his footing on the count of eight.
But the red-colored shorts of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was damaged quickly, heavy swelling appearing under his face as the pair landed hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur edged the decision with scoring tallies of even score, 115-113 and clearer margin, but both fighters were relentless and brave, all the qualities their hero Hatton stood for.
