UK light heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a close ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a fight where the techniques never quite clicked, with constant grabbing spoiling the contest.
The officials had it 96-94 for to the winner, with the third a 95-95.
"It wasn't my greatest performance but how do champions respond? They win," Buatsi commented in the ring as he was greeted with boos from the spectators.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts thought Parker, 31, should have been awarded the win but the contender suffered the second loss of his 28-match career.
Buatsi Earns Victory in Slow Affair
His career development has stalled in recent times through contract problems and extended periods of lack of fights.
Once tipped as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a last chance - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a solid defense to barely take the better rounds.
The spectators' acceptance decreased amid constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
Parker closed well, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding local rivalry battle to secure the European light-heavyweight belt.
The stadium grew hushed for a deeply poignant homage ahead of the contest as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the premier televised match card in the location since Hatton's passing. As the last bell faded, the stillness transitioned to a roar of "there's only one Ricky Hatton".
The fighting within the ring equaled the sentiment. The eventual winner was sent to the canvas by a powerful punch directly to the jaw in the initial stage, getting up unsteadily to his feet on the eight count.
But the traditional red attire of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was damaged quickly, noticeable damage forming under his eye as the duo traded punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur narrowly won the verdict with official scores of even score, close margin and clearer margin, but each competitor were persistent and full of heart, exactly what their inspiration the legend represented.
