UK light-heavyweight Buatsi secured a victory yet disappointing start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling individual punches in a fight where the approaches never quite clicked, with constant grabbing marring the action.
The officials had it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my greatest display but how do champions respond? They win," Buatsi commented in the arena as he was greeted with disapproval from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th career victory.
Several boxing experts felt Parker, 31, should have been awarded the decision but the British boxer endured the second loss of his 28-bout professional journey.
The Champion Secures Victory in Uneventful Affair
Buatsi's progress has plateaued in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to realize his potential - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers ready to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a solid defense to edge the more effective periods.
The spectators' acceptance wore thin because of regular grabbing, with Parker going down several times during grappling.
The underdog ended stronger, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester contest to capture the European light-heavyweight title.
The arena fell silent for a profoundly moving homage ahead of the contest as 10 bells tolled in honor of Manchester's own Hatton, who passed away 46 years old in last month.
It was the first major televised boxing event in the urban center since Hatton's death. As the concluding sound faded, the silence became a chant of "there's just one Ricky Hatton".
The action inside the ropes equaled the emotion. Arthur was sent to the canvas by a left hook squarely on the face in the initial stage, standing uncertainly to his position on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured quickly, significant bruising appearing under his eye as the pair exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
The victor narrowly won the verdict with official scores of 114-114, 115-113 and wider gap, but both fighters were determined and courageous, exactly what their idol Hatton embodied.
