British light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a close ten-round fight at the Manchester venue.
But Parker found success, delivering effective individual punches in a fight where the approaches never quite clicked, with constant grabbing disrupting the contest.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my greatest display but what do champions do? They succeed," Buatsi said in the arena as he was met by disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a 20th pro success.
Various analysts felt Parker, 31, should have been awarded the win but the contender experienced the second setback of his 28-match professional journey.
Buatsi Claims Win in Slow Encounter
The fighter's advancement has slowed in recent times through promotional disputes and considerable time of lack of fights.
Earlier forecast as the future of UK boxing, he now is positioned trying to achieve expectations - maybe in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers willing to be aggressive in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to barely take the better rounds.
The spectators' acceptance decreased during constant clinching, with the challenger falling to the floor several times during clinches.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the secondary fight, Arthur defeated Bradley Rea by divided decision in a terrific all-Manchester fight to win the continental title.
The venue fell silent for a extremely emotional tribute before the fight as ten bells rang in remembrance of the city's native Hatton, who died at age 46 in last month.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the final chime ended, the silence gave way to a chant of "there's just one Hatton".
The action during the bout matched the feeling. Arthur was knocked down by a left hook directly to the jaw in the second round, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was injured soon, heavy swelling forming under his eye as the fighters exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the decision with judges scorecards of 114-114, narrow lead and 115-112, but the two athletes were persistent and courageous, exactly what their idol the legend embodied.
