UK light-heavyweight Buatsi secured a victory yet disappointing debut to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful clean blows in a contest where the techniques didn't complement each other, with constant grabbing marring the fight.
The scoring judges scored it 96-94 to the winner, with the third a even score.
"It wasn't my greatest display but what defines champions? They win," Buatsi commented in the ring as he was greeted with boos from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts thought the challenger, thirty-one, should have been awarded the win but the contender suffered the second setback of his 28-match professional journey.
The Champion Earns Triumph in Uneventful Encounter
His career development has stalled in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to realize his potential - possibly in what feels like a last chance - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second bout at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a tight guard to barely take the more effective rounds.
The audience's tolerance diminished during frequent holding, with the challenger going down multiple occasions during grappling.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, former world champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Homage Motivates All-Manchester Contest
In the co-main event, Arthur beat Bradley Rea by divided decision in a terrific Manchester vs Manchester contest to secure the European light-heavyweight championship.
The venue grew hushed for a deeply poignant homage ahead of the contest as a bell ceremony sounded in remembrance of local hero Hatton, who passed away aged 46 in last month.
It was the first major televised fight night in the city since Hatton's passing. As the concluding sound faded, the silence transitioned to a roar of "there's only one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was sent to the canvas by a powerful punch flush on the face in the second round, rising shakily to his position on the count of eight.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up soon, significant bruising developing under his eye as the fighters traded powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the action hardly diminished over the full distance.
Arthur barely secured the result with official scores of even score, 115-113 and 115-112, but both fighters were determined and full of heart, all the qualities their hero Hatton embodied.
