UK light-heavyweight Buatsi made a winning yet disappointing debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But Parker had his moments, delivering effective individual punches in a contest where the approaches never quite clicked, with constant grabbing spoiling the contest.
The officials scored it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best performance but what defines champions? They win," Buatsi said in the ring as he was greeted with negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter experienced the second setback of his 28-bout career.
Buatsi Claims Victory in Uneventful Match
His career development has plateaued in the past few years through contract problems and considerable time of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a last chance - under recent support.
He opened tentatively, with both competitors willing to take risks in a cagey opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who operated with a protective stance to narrowly win the cleaner stanzas.
The audience's tolerance decreased during regular grabbing, with Parker going down various instances during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the likes of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Motivates Local Rivalry Fight
In the co-main event, Arthur beat Rea by close verdict in a excellent all-Manchester fight to capture the European light-heavyweight belt.
The stadium fell silent for a profoundly moving memorial ahead of the contest as a bell ceremony rang in memory of local hero Hatton, who passed away at age 46 in recently.
It was the premier televised boxing event in the location since Hatton's passing. As the last bell faded, the quiet became a chant of "there's only one Hatton".
The action inside the ropes matched the feeling. Arthur was knocked down by a powerful punch squarely on the jaw in the early round, rising shakily to his feet on the eight count.
But the red-colored equipment of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his face as the pair traded powerful shots.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over the full distance.
Arthur barely secured the verdict with scoring tallies of 114-114, close margin and wider gap, but the two athletes were determined and courageous, exactly what their inspiration the legend embodied.
