British light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight ten-round bout at the Manchester venue.
But Parker found success, landing some telling individual punches in a fight where the approaches failed to mesh, with constant grabbing disrupting the contest.
The scoring judges scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest display but what defines champions? They win," Buatsi commented in the ringside as he was met by disapproval from the crowd.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Various analysts felt the challenger, 31, merited the win but the British boxer suffered the second loss of his 28-fight professional journey.
Buatsi Claims Triumph in Slow Encounter
His career development has stalled in the past few years through management issues and long spells of lack of fights.
Earlier forecast as the future of the British fight scene, he now finds himself trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter prepared to overcommit in a cagey initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who worked behind a protective stance to edge the better periods.
The audience's tolerance wore thin amid constant clinching, with Parker falling to the floor several times during grappling.
The underdog ended stronger, but many rounds were difficult to score. However, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the secondary fight, Arthur beat Bradley Rea by divided decision in a terrific all-Manchester contest to secure the European light-heavyweight championship.
The venue grew hushed for a profoundly moving homage before the fight as ten bells rang in memory of the city's native Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the final chime disappeared, the stillness became a roar of "only one Hatton".
The action inside the ropes reflected the sentiment. Arthur was floored by a powerful punch flush on the face in the early round, rising shakily to his feet on the referee's count.
But the traditional red equipment of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was injured quickly, noticeable damage appearing under his orbital area as the fighters exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over the full distance.
Arthur edged the decision with scoring tallies of draw, 115-113 and wider gap, but both fighters were persistent and courageous, all the qualities their hero Hatton stood for.
