UK light-heavyweight Buatsi earned a win yet disappointing start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a tight ten-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful single shots in a fight where the styles didn't complement each other, with frequent clinching disrupting the action.
The scoring judges judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my greatest showing but what do champions do? They succeed," Buatsi said in the ringside as he was faced negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Various analysts thought Parker, 31, deserved the victory but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
The Champion Claims Triumph in Uneventful Encounter
Buatsi's progress has stalled in recent times through contract problems and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter ready to take risks in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to narrowly win the better rounds.
The spectators' acceptance diminished because of regular grabbing, with Parker going down various instances during grappling.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just a single round.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to secure the continental title.
The stadium grew hushed for a profoundly moving memorial before the fight as ten bells sounded in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound ended, the quiet transitioned to a chant of "there's just one Hatton".
The competition inside the ropes matched the sentiment. Arthur was sent to the canvas by a left hook directly to the chin in the early round, rising shakily to his footing on the referee's count.
But the traditional red attire of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was damaged soon, heavy swelling developing under his orbital area as the duo landed hard punches.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor narrowly won the result with official scores of 114-114, 115-113 and 115-112, but the two athletes were determined and courageous, everything their inspiration the legend represented.
