UK light heavyweight Buatsi secured a victory yet unimpressive debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful individual punches in a match where the styles failed to mesh, with excessive holding spoiling the action.
Two judges judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest showing but what defines champions? They succeed," Buatsi said in the ringside as he was met by negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Some ringside observers thought Parker, 31, deserved the decision but the contender endured the second defeat of his 28-match professional journey.
The Champion Earns Victory in Dull Encounter
The fighter's advancement has slowed in recent times through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to realize his potential - possibly in what feels like a final roll of the dice - under fresh management.
He started carefully, with neither fighter prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a protective stance to barely take the better periods.
The spectators' acceptance diminished because of frequent holding, with Parker going down multiple occasions during grappling.
Parker finished the stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor moves on, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Contest
In the supporting bout, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to secure the continental championship.
The arena grew hushed for a deeply poignant memorial prior to the bout as a bell ceremony rang in memory of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the premier televised fight night in the urban center since Hatton's passing. As the concluding sound faded, the stillness transitioned to a chant of "there's just one Hatton".
The competition inside the ropes reflected the emotion. The eventual winner was floored by a powerful punch directly to the chin in the early round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was marked up early, significant bruising appearing under his face as the duo landed punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur barely secured the decision with official scores of draw, narrow lead and 115-112, but the two athletes were persistent and brave, exactly what their inspiration Hatton embodied.
