British light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close 10-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a fight where the approaches didn't complement each other, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my best display but how do champions respond? They prevail," the victor said in the ring 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 last winter - to secure a 20th career victory.
Several boxing experts felt the challenger, thirty-one, deserved the decision but the British boxer suffered the second defeat of his 28-match professional journey.
Buatsi Claims Triumph in Dull Encounter
His career development has slowed in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the future of the British fight scene, he now is positioned trying to realize his potential - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter ready to overcommit in a calculated opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to narrowly win the more effective periods.
The crowd's patience decreased during constant clinching, with the challenger going down multiple occasions during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Hatton Homage Inspires Local Rivalry Contest
In the co-main event, Arthur beat Rea by divided decision in a outstanding local rivalry fight to win the European light heavyweight belt.
The venue grew hushed for a profoundly moving tribute before the fight as ten bells sounded in memory of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the final chime ended, the silence transitioned to a roar of "only one Hatton".
The competition within the ring reflected the sentiment. Arthur was floored by a significant blow directly to the jaw in the second round, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was injured early, noticeable damage developing under his orbital area as the duo exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the pace seldom decreased over 12 rounds.
Arthur edged the result with official scores of 114-114, narrow lead and 115-112, but both fighters were relentless and brave, all the qualities their idol the legend stood for.
