British light-heavyweight Buatsi made a winning yet disappointing start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the approaches never quite clicked, with excessive holding disrupting the contest.
The officials scored it 96-94 to the winner, with the third a even score.
"It wasn't my greatest performance but what do champions do? They prevail," Buatsi stated in the arena as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Several boxing experts felt Parker, thirty-one, should have been awarded the decision but the contender experienced the second setback of his 28-match professional journey.
The Champion Secures Win in Dull Match
His career development has stalled in recent years through management issues and long spells of inactivity.
Previously predicted as the future of British boxing, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors ready to overcommit in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second appearance at light-heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who operated with a protective stance to barely take the cleaner stanzas.
The spectators' acceptance decreased during frequent holding, with Parker going down various instances during close exchanges.
Parker closed well, but several stanzas were hard to judge. Although, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor advances, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a excellent all-Manchester fight to capture the European light-heavyweight championship.
The arena became quiet for a profoundly moving tribute before the fight as ten bells sounded in remembrance of local hero Ricky Hatton, who died 46 years old in recently.
It was the first major televised boxing event in the urban center since Hatton's death. As the last bell ended, the stillness transitioned to a shout of "only one Hatton".
The action during the bout matched the emotion. Arthur was knocked down by a powerful punch directly to the face in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged soon, heavy swelling forming under his eye as the pair exchanged punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the action rarely dropped over twelve rounds.
The victor barely secured the result with official scores of draw, 115-113 and 115-112, but both fighters were determined and brave, everything their hero the legend stood for.
