British light heavyweight Buatsi made a winning yet unimpressive beginning 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 judges' verdict in a tight 10-round fight at the boxing arena.
But Parker found success, landing some telling single shots in a match where the approaches failed to mesh, with constant grabbing spoiling the contest.
Two judges had it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best performance but what defines champions? They win," the victor said in the arena as he was greeted with disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, deserved the victory but the contender endured the second setback of his 28-fight professional journey.
The Champion Secures Win in Slow Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a last chance - under recent support.
He started carefully, with the two boxers prepared to take risks in a cagey opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second appearance at light heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the cleaner periods.
The audience's tolerance decreased because of regular grabbing, with Parker slipping to the canvas several times during clinches.
Parker closed well, but several stanzas were difficult to score. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires Local Rivalry Battle
In the co-main event, Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester contest to win the European light-heavyweight championship.
The venue fell silent for a extremely emotional homage ahead of the contest as ten bells rang in honor of local hero Hatton, who passed away at age 46 in September.
It was the premier televised fight night in the city since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a shout of "there's only one Hatton".
The action within the ring equaled the sentiment. Arthur was knocked down by a significant blow directly to the face in the second round, getting up unsteadily to his position on the referee's count.
But the traditional red attire of the fighter soon pressed ahead again. Across the ring, the opponent - in competitive blue - was injured quickly, significant bruising developing under his face as the duo exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over the full distance.
Arthur edged the decision with judges scorecards of even score, narrow lead and 115-112, but each competitor were determined and full of heart, all the qualities their idol Hatton stood for.
