UK light-heavyweight Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the techniques never quite clicked, with constant grabbing spoiling the contest.
The scoring judges judged it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest showing but how do champions respond? They prevail," Buatsi commented in the ringside as he was greeted with negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Several boxing experts thought Parker, 31, deserved the decision but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
The Champion Earns Triumph in Slow Encounter
His career development has slowed in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a last chance - under recent support.
He opened tentatively, with neither fighter ready to take risks in a cagey early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second fight at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who operated with a protective stance to barely take the cleaner rounds.
The crowd's patience decreased during regular grabbing, with Parker slipping to the canvas various instances during grappling.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Motivates Local Rivalry Fight
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester contest to win the European light heavyweight belt.
The arena grew hushed for a profoundly moving tribute ahead of the contest as ten bells sounded in honor of the city's native Hatton, who died at age 46 in recently.
It was the premier televised boxing event in the urban center since Hatton's passing. As the final chime ended, the silence transitioned to a roar of "there's just one Hatton".
The competition within the ring reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch directly to the face in the second round, rising shakily to his feet on the count of eight.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged early, noticeable damage appearing under his orbital area as the duo traded hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the verdict with scoring tallies of 114-114, close margin and clearer margin, but both fighters were determined and courageous, exactly what their hero Hatton embodied.
