UK light heavyweight Buatsi secured a victory yet unimpressive debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a close ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the approaches never quite clicked, with excessive holding disrupting the contest.
Two judges judged it 96-94 for to Buatsi, with the third a even score.
"It wasn't my finest showing but what defines champions? They succeed," the victor said in the ringside as he was faced disapproval from the crowd.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Various analysts felt the challenger, 31, should have been awarded the victory but the British boxer suffered the second loss of his 28-bout career.
The Champion Claims Triumph in Slow Affair
The fighter's advancement has plateaued in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He began cautiously, with neither fighter prepared to take risks in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second fight at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who worked behind a tight guard to narrowly win the more effective periods.
The crowd's patience wore thin amid constant clinching, with Parker going down several times during close exchanges.
Parker ended stronger, but many rounds were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Fight
In the secondary fight, Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester contest to win the European light heavyweight title.
The stadium became quiet for a deeply poignant memorial prior to the bout as a bell ceremony rang in honor of the city's native Hatton, who died aged 46 in September.
It was the first major televised fight night in the city since the passing of Hatton. As the last bell disappeared, the stillness became a roar of "there's just one Ricky Hatton".
The action inside the ropes reflected the feeling. Arthur was sent to the canvas by a left hook squarely on the face in the initial stage, rising shakily to his position on the count of eight.
But the traditional red attire of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged soon, noticeable damage developing under his orbital area as the pair landed hard punches.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur narrowly won the result with official scores of 114-114, narrow lead and wider gap, but each competitor were relentless and full of heart, everything their hero Hatton stood for.
