British light heavyweight Buatsi made a winning yet unimpressive debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a close 10-round fight at the Manchester venue.
But Parker had his moments, delivering effective single shots in a fight where the approaches never quite clicked, with excessive holding spoiling the fight.
The scoring judges had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest performance but what do champions do? They prevail," Buatsi commented in the ringside as he was faced boos from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers felt Parker, thirty-one, merited the victory but the contender experienced the second loss of his 28-fight professional journey.
The Champion Claims Victory in Slow Encounter
The fighter's advancement has stalled in the past few years through contract problems and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to achieve expectations - possibly in what seems like a final roll of the dice - under recent support.
He began cautiously, with neither fighter ready to be aggressive in a cagey opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who operated with a tight guard to edge the more effective rounds.
The spectators' acceptance decreased because of regular grabbing, with Parker falling to the floor several times during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Fight
In the co-main event, Arthur overcame Rea by divided decision in a terrific all-Manchester contest to capture the European light heavyweight title.
The stadium fell silent for a extremely emotional memorial before the fight as ten bells sounded in memory of local hero Hatton, who succumbed at age 46 in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound ended, the stillness transitioned to a chant of "only one Hatton".
The competition inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a left hook flush on the chin in the initial stage, rising shakily to his footing on the count of eight.
But the red-colored attire of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was marked up quickly, heavy swelling developing under his face as the duo landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the action rarely dropped over 12 rounds.
The victor barely secured the decision with official scores of draw, 115-113 and wider gap, but the two athletes were persistent and brave, everything their inspiration Hatton stood for.
