UK light heavyweight Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the approaches didn't complement each other, with frequent clinching spoiling the action.
Two judges had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest performance but how do champions respond? They succeed," the victor said in the arena as he was greeted with negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Various analysts believed Parker, 31, merited the victory but the contender endured the second setback of his 28-bout professional journey.
Buatsi Secures Triumph in Slow Encounter
Buatsi's progress has slowed in recent years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to realize his potential - perhaps in what appears to be a last chance - under fresh management.
He opened tentatively, with the two boxers willing to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who worked behind a tight guard to narrowly win the better rounds.
The audience's tolerance diminished because of constant clinching, with Parker falling to the floor multiple occasions during grappling.
The underdog ended stronger, but many rounds were challenging to assess. However, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Motivates Local Rivalry Fight
In the co-main event, Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester contest to win the continental belt.
The stadium grew hushed for a extremely emotional tribute ahead of the contest as 10 bells sounded in honor of local hero Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the final chime faded, the stillness became a chant of "only one Hatton".
The action within the ring matched the sentiment. Arthur was sent to the canvas by a left hook squarely on the chin in the second round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was marked up quickly, heavy swelling developing under his face as the fighters traded punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over the full distance.
The victor edged the decision with official scores of draw, close margin and clearer margin, but each competitor were persistent and courageous, exactly what their inspiration Hatton embodied.
