British light heavyweight Buatsi earned a win yet underwhelming beginning to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger found success, connecting with powerful clean blows in a contest where the styles failed to mesh, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 to the winner, with the final judge a even score.
"It wasn't my finest display but what do champions do? They win," the victor said in the arena as he was greeted with boos from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Various analysts believed Parker, thirty-one, should have been awarded the win but the British boxer endured the second loss of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Affair
His career development has slowed in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers willing to be aggressive in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who worked behind a solid defense to barely take the cleaner rounds.
The spectators' acceptance diminished because of regular grabbing, with Parker going down various instances during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires All-Manchester Battle
In the co-main event, Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester battle to capture the continental title.
The venue became quiet for a deeply poignant memorial before the fight as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised fight night in the urban center since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The fighting within the ring matched the sentiment. The eventual winner was knocked down by a left hook squarely on the face in the initial stage, getting up unsteadily to his position on the eight count.
But the red-colored equipment of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was damaged soon, heavy swelling forming under his orbital area as the duo traded powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the pace rarely dropped over twelve rounds.
Arthur barely secured the result with judges scorecards of even score, 115-113 and clearer margin, but both fighters were relentless and brave, all the qualities their inspiration Hatton stood for.
