UK light-heavyweight Buatsi made a winning yet disappointing beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the approaches didn't complement each other, with constant grabbing disrupting the contest.
Two judges judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best showing but what defines champions? They succeed," the victor said in the ringside as he was met by disapproval from the audience.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts believed Parker, 31, should have been awarded the win but the British boxer endured the second defeat of his 28-fight career.
The Champion Earns Triumph in Slow Affair
Buatsi's progress has plateaued in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter prepared to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to barely take the better rounds.
The audience's tolerance decreased amid regular grabbing, with Parker falling to the floor several times during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Hatton Memorial Motivates Manchester Derby Fight
In the co-main event, Arthur overcame Rea by close verdict in a excellent local rivalry contest to capture the European light heavyweight belt.
The venue fell silent for a extremely emotional homage prior to the bout as 10 bells rang in honor of the city's native Hatton, who succumbed aged 46 in recently.
It was the first major televised fight night in the urban center since Hatton's death. As the concluding sound disappeared, the quiet transitioned to a chant of "there's just one Hatton".
The fighting inside the ropes reflected the emotion. Arthur was sent to the canvas by a significant blow squarely on the face in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red attire of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his face as the pair landed punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the pace hardly diminished over 12 rounds.
The victor edged the decision with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were determined and brave, all the qualities their idol the legend stood for.
