British light-heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a tight 10-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the techniques never quite clicked, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what do champions do? They win," Buatsi stated in the arena as he was greeted with boos from the crowd.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, merited the win but the British boxer suffered the second loss of his 28-bout professional journey.
Buatsi Earns Triumph in Dull Match
Buatsi's progress has slowed in recent years through contract problems and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to realize his potential - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to overcommit in a tentative opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second bout at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to narrowly win the cleaner periods.
The audience's tolerance wore thin amid frequent holding, with the challenger going down multiple occasions during grappling.
Parker ended stronger, but many rounds were difficult to score. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Hatton Homage Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester battle to secure the continental title.
The stadium fell silent for a profoundly moving tribute prior to the bout as ten bells rang in honor of local hero Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound faded, the stillness gave way to a roar of "there's just one Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was knocked down by a powerful punch directly to the face in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of the fighter soon came forward again. Across the ring, the opponent - in opposing blue - was marked up soon, heavy swelling developing under his eye as the duo exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over the full distance.
The victor edged the verdict with scoring tallies of even score, 115-113 and wider gap, but each competitor were persistent and courageous, all the qualities their inspiration Hatton embodied.
