British light-heavyweight Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective clean blows in a fight where the approaches didn't complement each other, with constant grabbing marring the fight.
Two judges judged it 96-94 to the winner, with the third a even score.
"It wasn't my finest performance but what do champions do? They succeed," the victor stated in the ringside as he was faced negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers thought Parker, thirty-one, deserved the decision but the British boxer experienced the second defeat of his 28-match career.
Buatsi Secures Victory in Slow Affair
Buatsi's progress has slowed in recent times through management issues and long spells of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what feels like a last chance - under recent support.
He started carefully, with the two boxers willing to overcommit in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who worked behind a solid defense to edge the cleaner periods.
The crowd's patience wore thin amid frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific local rivalry contest to secure the continental championship.
The stadium became quiet for a extremely emotional tribute before the fight as ten bells rang in memory of local hero Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's death. As the concluding sound disappeared, the quiet gave way to a shout of "only one Ricky Hatton".
The fighting during the bout reflected the emotion. Arthur was sent to the canvas by a significant blow flush on the jaw in the second round, rising shakily to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged soon, significant bruising developing under his face as the duo landed hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity rarely dropped over twelve rounds.
The victor edged the verdict with judges scorecards of even score, narrow lead and clearer margin, but both fighters were determined and brave, exactly what their inspiration the legend stood for.
