UK light heavyweight Buatsi made a winning yet unimpressive beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a fight where the styles never quite clicked, with excessive holding marring the fight.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest performance but how do champions respond? They win," Buatsi stated in the ring as he was greeted with boos from the crowd.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought Parker, thirty-one, should have been awarded the victory but the British boxer suffered the second setback of his 28-fight professional journey.
The Champion Secures Victory in Dull Affair
Buatsi's progress has stalled in recent years through contract problems and long spells of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to achieve expectations - possibly in what seems like a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to narrowly win the better periods.
The crowd's patience diminished because of constant clinching, with Parker falling to the floor several times during grappling.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor progresses, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Motivates Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Rea by close verdict in a outstanding local rivalry battle to capture the European light heavyweight championship.
The stadium fell silent for a deeply poignant homage ahead of the contest as 10 bells tolled in remembrance of the city's native Hatton, who died 46 years old in last month.
It was the first major televised fight night in the urban center since the passing of Hatton. As the concluding sound faded, the quiet transitioned to a chant of "there's just one Hatton".
The competition inside the ropes matched the emotion. The eventual winner was floored by a powerful punch squarely on the face in the initial stage, getting up unsteadily to his footing on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was damaged quickly, heavy swelling developing under his face as the pair traded punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over the full distance.
Arthur barely secured the verdict with official scores of draw, close margin and clearer margin, but both fighters were determined and courageous, all the qualities their idol the legend stood for.
