British light heavyweight Buatsi made a winning yet disappointing debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a close 10-round fight at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a contest where the techniques never quite clicked, with excessive holding disrupting the contest.
The scoring judges had it 96-94 to the winner, with the third a 95-95.
"It wasn't my greatest showing but what do champions do? They prevail," the victor stated in the ring as he was greeted with boos from the spectators.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts felt the challenger, thirty-one, should have been awarded the decision but the contender suffered the second setback of his 28-fight career.
Buatsi Secures Triumph in Uneventful Affair
His career development has plateaued in recent times through contract problems and considerable time of lack of fights.
Once tipped as the next star of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with the two boxers prepared to overcommit in a tentative initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who utilized a solid defense to narrowly win the cleaner stanzas.
The crowd's patience decreased because of frequent holding, with Parker going down several times during clinches.
Parker closed well, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a outstanding local rivalry contest to secure the continental title.
The arena became quiet for a deeply poignant tribute ahead of the contest as a bell ceremony sounded in honor of the city's native Hatton, who passed away at age 46 in September.
It was the premier televised fight night in the city since Hatton's passing. As the concluding sound ended, the silence became a roar of "there's just one Ricky Hatton".
The competition inside the ropes equaled the emotion. The eventual winner was floored by a powerful punch squarely on the face in the early round, rising shakily to his feet on the referee's count.
But the red-colored shorts of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was damaged quickly, noticeable damage developing under his orbital area as the duo traded powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur edged the decision with official scores of draw, narrow lead and clearer margin, but both fighters were determined and courageous, all the qualities their hero Hatton stood for.
