UK light heavyweight Buatsi earned a win yet disappointing start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a split decision in a tight 10-round bout at the boxing arena.
But the challenger had his moments, connecting with powerful individual punches in a fight where the techniques never quite clicked, with excessive holding spoiling the contest.
Two judges had it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest showing but what defines champions? They succeed," the victor stated in the ringside as he was faced disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Several boxing experts thought Parker, 31, deserved the victory but the contender suffered the second loss of his 28-fight professional journey.
Buatsi Earns Triumph in Slow Encounter
Buatsi's progress has plateaued in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under recent support.
He began cautiously, with neither fighter prepared to take risks in a calculated initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to narrowly win the cleaner stanzas.
The audience's tolerance decreased during frequent holding, with the challenger slipping to the canvas various instances during grappling.
Parker closed well, but many rounds were hard to judge. Although, ex-champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Homage Inspires All-Manchester Battle
In the supporting bout, Arthur beat Rea by split-decision in a outstanding all-Manchester contest to capture the European light-heavyweight championship.
The stadium became quiet for a deeply poignant tribute before the fight as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the first major televised fight night in the city since Hatton's death. As the final chime disappeared, the stillness became a shout of "there's just one Ricky Hatton".
The competition within the ring matched the sentiment. Arthur was knocked down by a powerful punch directly to the chin in the initial stage, standing uncertainly to his feet on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was injured early, significant bruising developing under his orbital area as the duo exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor edged the result with judges scorecards of draw, close margin and wider gap, but both fighters were persistent and brave, all the qualities their inspiration Hatton embodied.
