UK light-heavyweight Buatsi made a winning yet disappointing start to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a competitive ten-round fight at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a fight where the approaches didn't complement each other, with excessive holding marring the contest.
The officials had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my finest display but what do champions do? They succeed," Buatsi said in the arena as he was greeted with disapproval from the crowd.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Some ringside observers thought the challenger, thirty-one, deserved the victory but the contender suffered the second loss of his 28-match professional journey.
Buatsi Earns Win in Slow Affair
His career development has plateaued in recent times through management issues and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter prepared to overcommit in a cagey initial rounds.
Parker, 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 bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a solid defense to edge the better periods.
The audience's tolerance decreased because of constant clinching, with the challenger going down multiple occasions during clinches.
Parker ended stronger, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a terrific all-Manchester contest to secure the European light heavyweight title.
The stadium fell silent for a deeply poignant tribute ahead of the contest as 10 bells tolled in honor of local hero Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the concluding sound ended, the stillness became a chant of "there's just one Hatton".
The fighting during the bout reflected the emotion. Arthur was sent to the canvas by a left hook squarely on the jaw in the initial stage, rising shakily to his position on the referee's count.
But the traditional red attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was damaged quickly, significant bruising developing under his eye as the duo landed powerful shots.
The challenger, 27, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over the full distance.
The victor edged the decision with scoring tallies of 114-114, close margin and wider gap, but both fighters were persistent and courageous, everything their inspiration the legend stood for.
