UK light heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive 10-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a match where the techniques never quite clicked, with constant grabbing disrupting the action.
Two judges judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi stated in the ringside as he was met by boos from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Various analysts felt Parker, 31, should have been awarded the win but the contender suffered the second defeat of his 28-match career.
The Champion Earns Triumph in Dull Affair
Buatsi's progress has slowed in recent times through management issues and considerable time of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers ready to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to edge the more effective rounds.
The audience's tolerance wore thin because of frequent holding, with Parker falling to the floor several times during grappling.
Parker closed well, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a excellent all-Manchester battle to secure the European light heavyweight belt.
The venue became quiet for a deeply poignant tribute before the fight as a bell ceremony tolled in memory of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the first major televised boxing event in the city since Hatton's passing. As the final chime disappeared, the quiet transitioned to a roar of "there's only one Hatton".
The action within the ring matched the emotion. The eventual winner was sent to the canvas by a powerful punch flush on the jaw in the second round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was injured early, heavy swelling developing under his face as the fighters exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the pace hardly diminished over the full distance.
The victor barely secured the decision with judges scorecards of 114-114, close margin and clearer margin, but both fighters were persistent and brave, all the qualities their idol the legend embodied.
