British light-heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a match where the styles didn't complement each other, with frequent clinching spoiling the action.
The officials scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," the victor commented in the ringside as he was met by negative reactions from the audience.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts thought Parker, thirty-one, merited the victory but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
The Champion Secures Triumph in Uneventful Encounter
His career development has slowed in the past few years through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now finds himself trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under recent support.
He began cautiously, with neither fighter ready to take risks in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who utilized a protective stance to narrowly win the more effective rounds.
The audience's tolerance wore thin during constant clinching, with the challenger falling to the floor various instances during grappling.
The underdog closed well, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Tribute Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a terrific all-Manchester battle to capture the continental championship.
The arena fell silent for a profoundly moving memorial before the fight as 10 bells tolled in honor of the city's native Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised boxing event in the location since the passing of Hatton. As the final chime disappeared, the stillness became a shout of "only one Hatton".
The action during the bout equaled the sentiment. Arthur was knocked down by a left hook flush on the chin in the early round, standing uncertainly to his position on the referee's count.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was damaged early, significant bruising appearing under his face as the fighters traded punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity rarely dropped over 12 rounds.
Arthur edged the decision with judges scorecards of 114-114, close margin and clearer margin, but both fighters were relentless and brave, all the qualities their hero Hatton represented.
