British 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 bronze medallist's work rate likely gained him a split decision in a tight 10-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the techniques never quite clicked, with excessive holding marring the fight.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what do champions do? They succeed," the victor commented in the arena as he was greeted with disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Some ringside observers thought the challenger, thirty-one, deserved the win but the British boxer suffered the second defeat of his 28-fight professional journey.
Buatsi Claims Win in Slow Match
The fighter's advancement has stalled in the past few years through contract problems and extended periods of lack of fights.
Previously predicted as the future of UK boxing, he now finds himself trying to realize his potential - perhaps in what feels like a final roll of the dice - under recent support.
He began cautiously, with neither fighter ready to overcommit in a calculated early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who utilized a solid defense to barely take the cleaner rounds.
The audience's tolerance wore thin during constant clinching, with Parker slipping to the canvas various instances during clinches.
Parker closed well, but several stanzas were challenging to assess. However, former world champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Motivates Manchester Derby Fight
In the secondary fight, Arthur beat Rea by split-decision in a terrific all-Manchester fight to win the European light-heavyweight championship.
The arena grew hushed for a deeply poignant memorial before the fight as 10 bells sounded in memory of the city's native Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised fight night in the city since the passing of Hatton. As the final chime ended, the silence became a roar of "there's just one Hatton".
The fighting within the ring equaled the sentiment. Arthur was knocked down by a powerful punch directly to the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was marked up quickly, noticeable damage forming under his eye as the fighters exchanged hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the action rarely dropped over the full distance.
The victor barely secured the decision with official scores of 114-114, close margin and 115-112, but both fighters were relentless and courageous, all the qualities their inspiration Hatton stood for.
