UK light heavyweight Buatsi earned a win yet underwhelming debut to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close 10-round fight at the boxing arena.
But the challenger had his moments, connecting with powerful individual punches in a fight where the styles never quite clicked, with excessive holding marring the contest.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest display but what defines champions? They win," Buatsi said in the ring as he was met by negative reactions from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Some ringside observers thought Parker, 31, should have been awarded the victory but the contender suffered the second setback of his 28-bout professional journey.
The Champion Secures Win in Slow Match
Buatsi's progress has slowed in the past few years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under fresh management.
He began cautiously, with neither fighter prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the more effective stanzas.
The crowd's patience wore thin amid frequent holding, with Parker falling to the floor multiple occasions during clinches.
Parker closed well, but several stanzas were difficult to score. However, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Hatton Memorial Inspires Local Rivalry Contest
In the secondary fight, Arthur overcame Rea by close verdict in a terrific local rivalry contest to secure the European light-heavyweight championship.
The stadium became quiet for a deeply poignant tribute before the fight as 10 bells sounded in honor of Manchester's own Hatton, who passed away aged 46 in recently.
It was the first major televised boxing event in the city since Hatton's death. As the final chime faded, the stillness gave way to a chant of "there's just one Hatton".
The action inside the ropes equaled the emotion. The eventual winner was sent to the canvas by a left hook squarely on the face in the initial stage, rising shakily to his position on the referee's count.
But the red-colored shorts of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was injured early, heavy swelling appearing under his face as the fighters traded punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur edged the decision with official scores of even score, 115-113 and 115-112, but both fighters were persistent and courageous, all the qualities their inspiration Hatton represented.
