British light-heavyweight Buatsi earned a win yet disappointing start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close 10-round bout at the Manchester venue.
But Parker found success, delivering effective single shots in a contest where the styles never quite clicked, with excessive holding disrupting the fight.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi stated in the ringside as he was greeted with disapproval from the crowd.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts felt Parker, 31, should have been awarded the win but the British boxer endured the second defeat of his 28-match professional journey.
Buatsi Earns Triumph in Uneventful Encounter
His career development has slowed in recent times through contract problems and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors ready to overcommit in a cagey initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who operated with a tight guard to edge the better stanzas.
The audience's tolerance wore thin because of frequent holding, with Parker falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Motivates Local Rivalry Fight
In the co-main event, Arthur defeated Rea by split-decision in a excellent local rivalry contest to capture the continental title.
The arena fell silent for a deeply poignant memorial prior to the bout as ten bells rang in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the concluding sound ended, the silence gave way to a chant of "there's just one Ricky Hatton".
The competition inside the ropes reflected the emotion. Arthur was sent to the canvas by a significant blow squarely on the jaw in the early round, getting up unsteadily to his feet on the count of eight.
But the traditional red attire of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was damaged soon, heavy swelling forming under his orbital area as the pair traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor barely secured the decision with official scores of draw, 115-113 and wider gap, but both fighters were relentless and brave, exactly what their idol the legend embodied.
