British light-heavyweight Buatsi made a winning yet disappointing start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a fight where the approaches failed to mesh, with constant grabbing marring the fight.
Two judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest performance but what defines champions? They win," the victor commented in the ring as he was met by negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Some ringside observers believed Parker, thirty-one, deserved the win but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Encounter
His career development has plateaued in recent times through contract problems and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to realize his potential - possibly in what seems like a last chance - under recent support.
He opened tentatively, with both competitors prepared to take risks in a tentative early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to edge the better periods.
The spectators' acceptance wore thin amid regular grabbing, with Parker falling to the floor several times during close exchanges.
The underdog finished the stronger, but multiple periods were difficult to score. Nevertheless, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the co-main event, Arthur defeated Bradley Rea by divided decision in a outstanding local rivalry fight to win the continental belt.
The venue fell silent for a profoundly moving memorial prior to the bout as a bell ceremony tolled in honor of local hero Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the last bell faded, the stillness became a roar of "there's just one Hatton".
The action inside the ropes reflected the sentiment. The eventual winner was sent to the canvas by a significant blow flush on the chin in the early round, rising shakily to his feet on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up soon, heavy swelling appearing under his eye as the pair exchanged punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the action rarely dropped over 12 rounds.
Arthur barely secured the decision with official scores of even score, 115-113 and 115-112, but the two athletes were determined and courageous, exactly what their hero Hatton stood for.
