British light-heavyweight Buatsi secured a victory yet underwhelming debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a close ten-round fight at the boxing arena.
But Parker had his moments, delivering effective single shots in a match where the approaches never quite clicked, with excessive holding disrupting the fight.
The scoring judges scored it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They win," the victor commented in the ring as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers thought Parker, thirty-one, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-bout professional journey.
The Champion Earns Victory in Dull Encounter
Buatsi's progress has slowed in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under recent support.
He started carefully, with the two boxers willing to be aggressive in a tentative opening few rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who utilized a protective stance to edge the cleaner periods.
The spectators' acceptance decreased because of constant clinching, with Parker going down multiple occasions during clinches.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Local Rivalry Fight
In the secondary fight, Arthur beat Rea by divided decision in a terrific local rivalry battle to capture the European light heavyweight championship.
The arena fell silent for a extremely emotional memorial before the fight as a bell ceremony sounded in remembrance of the city's native Hatton, who died at age 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the last bell disappeared, the quiet gave way to a shout of "there's just one Hatton".
The action during the bout reflected the emotion. Arthur was sent to the canvas by a powerful punch directly to the chin in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was injured early, significant bruising developing under his orbital area as the duo landed punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the action hardly diminished over 12 rounds.
The victor edged the verdict with official scores of even score, 115-113 and wider gap, but each competitor were determined and full of heart, all the qualities their hero Hatton embodied.
