British light heavyweight Buatsi earned a win yet disappointing start to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the styles failed to mesh, with constant grabbing marring the action.
The scoring judges had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best performance but how do champions respond? They win," Buatsi stated in the ringside as he was faced boos from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts believed the challenger, thirty-one, merited the win but the Derbyshire fighter experienced the second defeat of his 28-bout professional journey.
Buatsi Secures Victory in Slow Encounter
Buatsi's progress has slowed in the past few years through contract problems and long spells of inactivity.
Previously predicted as the future of British boxing, he now is positioned trying to achieve expectations - possibly in what feels like a final roll of the dice - under recent support.
He started carefully, with both competitors willing to be aggressive in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand in the contest, was making just his second appearance at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for the favorite, who worked behind a solid defense to barely take the more effective periods.
The audience's tolerance decreased amid frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Tribute Fuels Manchester Derby Fight
In the co-main event, Arthur beat Rea by split-decision in a terrific all-Manchester battle to capture the continental title.
The venue fell silent for a profoundly moving memorial ahead of the contest as ten bells sounded in honor of the city's native Hatton, who died 46 years old in September.
It was the premier televised fight night in the urban center since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a shout of "there's just one Hatton".
The action inside the ropes equaled the sentiment. Arthur was knocked down by a left hook flush on the chin in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was injured early, heavy swelling forming under his orbital area as the fighters landed punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the decision with scoring tallies of even score, narrow lead and 115-112, but both fighters were persistent and brave, everything their idol Hatton represented.
