British light heavyweight Buatsi secured a victory yet underwhelming debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round contest at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a match where the approaches failed to mesh, with frequent clinching disrupting the contest.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my greatest display but what defines champions? They win," the victor commented in the ringside 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 his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers felt Parker, 31, should have been awarded the win but the contender endured the second defeat of his 28-fight professional journey.
Buatsi Claims Victory in Dull Affair
The fighter's advancement has plateaued in recent times through contract problems and extended periods of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - possibly in what seems like a last chance - under new promotional backing.
He started carefully, with the two boxers willing to be aggressive in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at light-heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who utilized a protective stance to barely take the cleaner rounds.
The audience's tolerance decreased because of frequent holding, with Parker falling to the floor various instances during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a outstanding all-Manchester fight to capture the European light-heavyweight championship.
The arena became quiet for a extremely emotional tribute ahead of the contest as a bell ceremony sounded in honor of the city's native Ricky Hatton, who died at age 46 in September.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The action during the bout reflected the sentiment. Arthur was knocked down by a left hook squarely on the face in the early round, standing uncertainly to his position on the eight count.
But the red-colored equipment of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his orbital area as the duo exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the pace seldom decreased over the full distance.
Arthur edged the result with official scores of draw, close margin and clearer margin, but each competitor were relentless and full of heart, all the qualities their inspiration Hatton embodied.
