UK light heavyweight Buatsi secured a victory yet disappointing start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round contest at the Manchester venue.
But the challenger found success, connecting with powerful single shots in a contest where the techniques didn't complement each other, with excessive holding marring the fight.
The officials had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest performance but what do champions do? They succeed," the victor said in the ringside as he was faced negative reactions from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts believed the challenger, 31, merited the victory but the contender endured the second loss of his 28-fight career.
The Champion Earns Win in Uneventful Match
Buatsi's progress has stalled in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to achieve expectations - perhaps in what seems like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers prepared to be aggressive in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to edge the cleaner rounds.
The spectators' acceptance wore thin during frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Contest
In the co-main event, Arthur beat Bradley Rea by divided decision in a terrific local rivalry battle to win the European light heavyweight title.
The venue grew hushed for a extremely emotional memorial ahead of the contest as 10 bells tolled in honor of Manchester's own Hatton, who succumbed aged 46 in September.
It was the premier televised fight night in the city since the passing of Hatton. As the final chime faded, the quiet became a roar of "there's just one Ricky Hatton".
The competition within the ring matched the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the chin in the early round, rising shakily to his feet 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 injured early, heavy swelling forming under his face as the duo traded powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity rarely dropped over the full distance.
The victor barely secured the result with scoring tallies of 114-114, 115-113 and wider gap, but the two athletes were persistent and courageous, exactly what their inspiration the legend embodied.
