British light-heavyweight Buatsi earned a win yet underwhelming debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight ten-round bout at the Manchester venue.
But Parker found success, landing some telling clean blows in a match where the techniques didn't complement each other, with excessive holding disrupting the action.
The officials judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my greatest display but how do champions respond? They win," the victor stated in the arena as he was met by boos from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers felt Parker, thirty-one, should have been awarded the decision but the British boxer endured the second setback of his 28-bout professional journey.
Buatsi Claims Triumph in Uneventful Encounter
The fighter's advancement has plateaued in recent times through management issues and considerable time of lack of fights.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - possibly in what seems like a last chance - under new promotional backing.
He opened tentatively, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second fight at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who operated with a solid defense to narrowly win the better rounds.
The audience's tolerance decreased amid frequent holding, with Parker going down multiple occasions during clinches.
Parker ended stronger, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Contest
In the co-main event, Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester fight to win the continental title.
The venue fell silent for a profoundly moving homage ahead of the contest as 10 bells rang in honor of the city's native Hatton, who passed away aged 46 in September.
It was the first major televised fight night in the city since the passing of Hatton. As the concluding sound disappeared, the stillness gave way to a chant of "only one Hatton".
The fighting inside the ropes reflected the feeling. Arthur was sent to the canvas by a powerful punch directly to the jaw in the second round, rising shakily to his position on the eight count.
But the traditional red attire of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was damaged early, noticeable damage forming under his face as the duo traded powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the pace hardly diminished over twelve rounds.
The victor narrowly won the result with judges scorecards of draw, 115-113 and wider gap, but each competitor were determined and full of heart, everything their idol Hatton embodied.
