UK light heavyweight Buatsi secured a victory yet underwhelming debut to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger had his moments, landing some telling clean blows in a contest where the styles didn't complement each other, with excessive holding marring the contest.
Two judges judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They succeed," the victor stated in the arena as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers felt Parker, thirty-one, deserved the victory but the British boxer suffered the second defeat of his 28-match career.
Buatsi Claims Victory in Uneventful Encounter
Buatsi's progress has plateaued in the past few years through management issues and long spells of inactivity.
Once tipped as the next star of British boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter ready to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a solid defense to edge the better periods.
The crowd's patience diminished because of frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Hatton Memorial Fuels Local Rivalry Contest
In the supporting bout, Arthur beat Bradley Rea by divided decision in a outstanding local rivalry battle to secure the European light heavyweight title.
The venue fell silent for a extremely emotional tribute prior to the bout as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who died 46 years old in September.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the last bell ended, the stillness transitioned to a chant of "there's only one Hatton".
The action within the ring reflected the emotion. The eventual winner was floored by a left hook squarely on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was damaged early, heavy swelling appearing under his eye as the fighters exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the action rarely dropped over the full distance.
The victor narrowly won the verdict with official scores of draw, close margin and 115-112, but each competitor were determined and brave, exactly what their idol Hatton represented.
