British light heavyweight Buatsi secured a victory yet disappointing debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a fight where the techniques didn't complement each other, with constant grabbing marring the fight.
The scoring judges judged it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They succeed," the victor stated in the arena as he was greeted with boos from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Various analysts thought the challenger, thirty-one, merited the win but the Derbyshire fighter suffered the second defeat of his 28-fight career.
The Champion Secures Win in Dull Match
Buatsi's progress has stalled in recent years through management issues and considerable time of lack of fights.
Previously predicted as the future of UK boxing, he now stands trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who utilized a tight guard to narrowly win the better periods.
The crowd's patience decreased during constant clinching, with Parker falling to the floor several times during grappling.
Parker closed well, but many rounds were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Hatton Memorial Motivates Manchester Derby Contest
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a terrific all-Manchester contest to secure the continental championship.
The venue grew hushed for a profoundly moving homage before the fight as ten bells rang in honor of Manchester's own Hatton, who passed away at age 46 in September.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound disappeared, the silence transitioned to a chant of "there's just one Hatton".
The competition within the ring equaled the emotion. The eventual winner was knocked down by a left hook flush on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured quickly, noticeable damage developing under his face as the duo landed punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the verdict with official scores of even score, narrow lead and 115-112, but both fighters were persistent and full of heart, everything their idol Hatton embodied.
