UK light heavyweight Buatsi made a winning yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a competitive ten-round bout at the Manchester venue.
But the challenger found success, delivering effective individual punches in a fight where the techniques failed to mesh, with frequent clinching disrupting the contest.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but what defines champions? They prevail," the victor stated in the ring as he was faced boos from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers believed Parker, thirty-one, deserved the win but the British boxer endured the second setback of his 28-fight professional journey.
The Champion Claims Triumph in Dull Match
Buatsi's progress has plateaued in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to realize his potential - possibly in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers willing to overcommit in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who worked behind a protective stance to narrowly win the better rounds.
The crowd's patience diminished because of constant clinching, with Parker falling to the floor several times during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Fuels Manchester Derby Fight
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester battle to win the continental championship.
The arena fell silent for a profoundly moving homage ahead of the contest as 10 bells sounded in remembrance of local hero Ricky Hatton, who died 46 years old in recently.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a shout of "there's just one Hatton".
The action within the ring reflected the emotion. Arthur was sent to the canvas by a left hook directly to the chin in the initial stage, getting up unsteadily to his feet on the count of eight.
But the traditional red attire of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was injured soon, heavy swelling developing under his orbital area as the fighters exchanged punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the result with judges scorecards of draw, 115-113 and clearer margin, but each competitor were determined and courageous, everything their inspiration Hatton represented.
