UK light-heavyweight Buatsi made a winning yet unimpressive start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight 10-round fight at the Manchester venue.
But Parker found success, delivering effective clean blows in a fight where the techniques failed to mesh, with frequent clinching marring the fight.
Two judges had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best display but what do champions do? They win," the victor commented in the arena as he was greeted with disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the victory but the British boxer experienced the second defeat of his 28-fight career.
Buatsi Earns Win in Dull Affair
Buatsi's progress has slowed in recent years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of UK boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under recent support.
He started carefully, with both competitors willing to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second fight at light heavyweight.
By the fight's midpoint, the underdog had kept it close, making life awkward for the favorite, who operated with a tight guard to narrowly win the better periods.
The audience's tolerance diminished because of constant clinching, with Parker falling to the floor several times during close exchanges.
The underdog closed well, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a excellent all-Manchester fight to secure the European light heavyweight title.
The arena became quiet for a extremely emotional tribute before the fight as 10 bells tolled in memory of Manchester's own Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the last bell disappeared, the quiet became a chant of "only one Hatton".
The fighting inside the ropes matched the sentiment. Arthur was sent to the canvas by a significant blow directly to the chin in the initial stage, getting up unsteadily to his position on the referee's count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was injured early, significant bruising appearing under his eye as the fighters landed hard punches.
Rea, 27, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the pace seldom decreased over the full distance.
The victor barely secured the verdict with official scores of even score, close margin and 115-112, but both fighters were determined and brave, exactly what their inspiration Hatton embodied.
