British light heavyweight Buatsi earned a win yet disappointing beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight 10-round contest at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a match where the approaches never quite clicked, with frequent clinching marring the action.
The scoring judges judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor stated in the ring as he was faced boos from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts thought Parker, 31, merited the victory but the contender suffered the second loss of his 28-bout professional journey.
The Champion Secures Win in Dull Match
The fighter's advancement has stalled in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to achieve expectations - possibly in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers ready to overcommit in a tentative initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second bout at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who utilized a protective stance to barely take the cleaner periods.
The audience's tolerance diminished amid constant clinching, with the challenger slipping to the canvas various instances during grappling.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a terrific all-Manchester contest to capture the European light-heavyweight title.
The arena became quiet for a profoundly moving homage prior to the bout as ten bells rang in memory of local hero Hatton, who passed away 46 years old in September.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the final chime faded, the stillness became a roar of "only one Hatton".
The fighting within the ring matched the sentiment. Arthur was sent to the canvas by a left hook directly to the jaw in the second round, rising shakily to his position on the referee's count.
But the red-colored attire of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was damaged early, noticeable damage appearing under his face as the duo traded punishing blows.
The challenger, 27, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the result with official scores of draw, 115-113 and clearer margin, but both fighters were relentless and brave, all the qualities their inspiration Hatton stood for.
