UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a competitive 10-round bout at the Manchester venue.
But the challenger had his moments, landing some telling single shots in a match where the approaches didn't complement each other, with excessive holding marring the fight.
The scoring judges had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best display but what defines champions? They prevail," Buatsi said in the ring as he was met by disapproval from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Several boxing experts felt Parker, thirty-one, should have been awarded the decision but the contender suffered the second setback of his 28-fight career.
Buatsi Claims Victory in Uneventful Encounter
The fighter's advancement has slowed in recent times through contract problems and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now is positioned trying to realize his potential - perhaps in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to overcommit in a tentative early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who operated with a protective stance to barely take the better periods.
The audience's tolerance diminished during constant clinching, with the challenger going down several times during grappling.
Parker finished the stronger, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Hatton Memorial Fuels Manchester Derby Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific Manchester vs Manchester battle to capture the continental championship.
The stadium became quiet for a profoundly moving homage before the fight as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the final chime faded, the silence gave way to a roar of "only one Hatton".
The action inside the ropes matched the emotion. The eventual winner was floored by a powerful punch directly to the face in the initial stage, rising shakily to his feet on the eight count.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his face as the fighters landed powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur edged the verdict with scoring tallies of draw, narrow lead and clearer margin, but the two athletes were determined and full of heart, everything their idol Hatton represented.
