UK light-heavyweight Buatsi made a winning yet disappointing beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive ten-round contest at the boxing arena.
But Parker found success, landing some telling clean blows in a match where the techniques didn't complement each other, with excessive holding spoiling the fight.
The officials had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest display but what defines champions? They win," the victor commented in the ring as he was greeted with disapproval from the audience.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers thought the challenger, 31, merited the win but the contender endured the second defeat of his 28-fight career.
Buatsi Secures Triumph in Dull Affair
His career development has stalled in recent times through contract problems and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - maybe in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers ready to take risks in a tentative initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who operated with a solid defense to edge the cleaner periods.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas various instances during clinches.
Parker ended stronger, but multiple periods were difficult to score. However, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the European light-heavyweight championship.
The stadium grew hushed for a profoundly moving tribute before the fight as ten bells tolled in memory of Manchester's own Hatton, who passed away aged 46 in recently.
It was the premier televised match card in the city since Hatton's passing. As the final chime faded, the silence gave way to a chant of "there's only one Hatton".
The action during the bout equaled the feeling. Arthur was floored by a powerful punch flush on the face in the early round, getting up unsteadily to his feet on the eight count.
But the red-colored shorts of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was damaged soon, significant bruising forming under his face as the pair traded hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the intensity rarely dropped over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of draw, close margin and wider gap, but the two athletes were relentless and courageous, all the qualities their inspiration Hatton stood for.
