UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful individual punches in a fight where the styles didn't complement each other, with excessive holding spoiling the action.
Two judges scored it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best showing but what do champions do? They win," Buatsi commented in the ringside as he was faced boos from the crowd.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts thought Parker, 31, should have been awarded the win but the contender suffered the second loss of his 28-bout professional journey.
The Champion Secures Triumph in Uneventful Encounter
Buatsi's progress has stalled in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under recent support.
He began cautiously, with both competitors willing to take risks in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was participating in his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who utilized a tight guard to narrowly win the more effective rounds.
The spectators' acceptance wore thin amid constant clinching, with Parker slipping to the canvas several times during grappling.
The underdog ended stronger, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Fuels Manchester Derby Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to capture the European light-heavyweight belt.
The venue grew hushed for a deeply poignant homage before the fight as 10 bells rang in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the concluding sound faded, the quiet became a roar of "there's just one Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the second round, standing uncertainly to his footing on the referee's count.
But the red-colored shorts of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was marked up quickly, significant bruising forming under his face as the pair exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over 12 rounds.
The victor barely secured the decision with scoring tallies of draw, 115-113 and clearer margin, but each competitor were persistent and brave, exactly what their inspiration Hatton represented.
