UK light heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a fight where the approaches didn't complement each other, with frequent clinching marring the fight.
The officials had it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best display but what do champions do? They win," the victor stated in the ring as he was met by disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts felt Parker, 31, merited the victory but the British boxer suffered the second loss of his 28-fight professional journey.
The Champion Secures Victory in Dull Match
His career development has stalled in the past few years through management issues and long spells of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with both competitors willing to take risks in a tentative early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who worked behind a tight guard to edge the cleaner rounds.
The spectators' acceptance wore thin amid constant clinching, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. Although, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Motivates Manchester Derby Battle
In the co-main event, Arthur overcame Bradley Rea by split-decision in a excellent all-Manchester battle to win the European light heavyweight championship.
The stadium became quiet for a deeply poignant homage before the fight as ten bells rang in remembrance of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the last bell faded, the stillness became a roar of "only one Ricky Hatton".
The action inside the ropes equaled the emotion. The eventual winner was knocked down by a left hook squarely on the jaw in the early round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red attire of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was marked up early, noticeable damage appearing under his orbital area as the duo exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur barely secured the decision with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were persistent and courageous, all the qualities their idol the legend represented.
